Closes #1442

```
WARNING: DATA RACE
Write at 0x00c4209de7c8 by goroutine 23:
  github.com/tendermint/tendermint/types.(*Block).fillHeader()
      /home/vagrant/go/src/github.com/tendermint/tendermint/types/block.go:88 +0x157
  github.com/tendermint/tendermint/types.(*Block).Hash()
      /home/vagrant/go/src/github.com/tendermint/tendermint/types/block.go:104 +0x121
  github.com/tendermint/tendermint/types.(*Block).HashesTo()
      /home/vagrant/go/src/github.com/tendermint/tendermint/types/block.go:135 +0x4f
  github.com/tendermint/tendermint/consensus.(*ConsensusState).enterPrecommit()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:1037 +0x182d
  github.com/tendermint/tendermint/consensus.(*ConsensusState).addVote()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:1425 +0x1a6c
  github.com/tendermint/tendermint/consensus.(*ConsensusState).tryAddVote()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:1318 +0x77
  github.com/tendermint/tendermint/consensus.(*ConsensusState).handleMsg()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:581 +0x7a9
  github.com/tendermint/tendermint/consensus.(*ConsensusState).receiveRoutine()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:539 +0x6c3

Previous read at 0x00c4209de7c8 by goroutine 47:
  github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common.(*HexBytes).MarshalJSON()
      <autogenerated>:1 +0x52
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.invokeMarshalJSON()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:433 +0x88
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec)._encodeReflectJSON()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:82 +0x8d2
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec).encodeReflectJSON()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:50 +0x10e
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec).encodeReflectJSONStruct()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:348 +0x539
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec)._encodeReflectJSON()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:119 +0x83f
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec).encodeReflectJSON()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:50 +0x10e
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec).encodeReflectJSONStruct()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:348 +0x539
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec)._encodeReflectJSON()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:119 +0x83f
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec).encodeReflectJSON()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:50 +0x10e
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec).encodeReflectJSONStruct()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:348 +0x539
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec)._encodeReflectJSON()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:119 +0x83f
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec).encodeReflectJSON()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:50 +0x10e
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec).encodeReflectJSONStruct()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:348 +0x539
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec)._encodeReflectJSON()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:119 +0x83f
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec).encodeReflectJSON()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/json-encode.go:50 +0x10e
  github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino.(*Codec).MarshalJSON()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/go-amino/amino.go:296 +0x182
  github.com/tendermint/tendermint/rpc/lib/types.NewRPCSuccessResponse()
      /home/vagrant/go/src/github.com/tendermint/tendermint/rpc/lib/types/types.go:100 +0x12c
  github.com/tendermint/tendermint/rpc/lib/server.makeJSONRPCHandler.func1()
      /home/vagrant/go/src/github.com/tendermint/tendermint/rpc/lib/server/handlers.go:152 +0xab7
  net/http.HandlerFunc.ServeHTTP()
      /usr/lib/go-1.9/src/net/http/server.go:1918 +0x51
  net/http.(*ServeMux).ServeHTTP()
      /usr/lib/go-1.9/src/net/http/server.go:2254 +0xa2
  github.com/tendermint/tendermint/rpc/lib/server.RecoverAndLogHandler.func1()
      /home/vagrant/go/src/github.com/tendermint/tendermint/rpc/lib/server/http_server.go:138 +0x4fa
  net/http.HandlerFunc.ServeHTTP()
      /usr/lib/go-1.9/src/net/http/server.go:1918 +0x51
  net/http.serverHandler.ServeHTTP()
      /usr/lib/go-1.9/src/net/http/server.go:2619 +0xbc
  net/http.(*conn).serve()
      /usr/lib/go-1.9/src/net/http/server.go:1801 +0x83b

Goroutine 23 (running) created at:
  github.com/tendermint/tendermint/consensus.(*ConsensusState).OnStart()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:250 +0x35b
  github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common.(*BaseService).Start()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common/service.go:130 +0x5fc
  github.com/tendermint/tendermint/consensus.(*ConsensusReactor).OnStart()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/reactor.go:69 +0x1b4
  github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common.(*BaseService).Start()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common/service.go:130 +0x5fc
  github.com/tendermint/tendermint/consensus.(*ConsensusReactor).Start()
      <autogenerated>:1 +0x43
  github.com/tendermint/tendermint/p2p.(*Switch).OnStart()
      /home/vagrant/go/src/github.com/tendermint/tendermint/p2p/switch.go:177 +0x124
  github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common.(*BaseService).Start()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common/service.go:130 +0x5fc
  github.com/tendermint/tendermint/node.(*Node).OnStart()
      /home/vagrant/go/src/github.com/tendermint/tendermint/node/node.go:416 +0xa1b
  github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common.(*BaseService).Start()
      /home/vagrant/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common/service.go:130 +0x5fc
  github.com/tendermint/tendermint/rpc/test.StartTendermint()
      /home/vagrant/go/src/github.com/tendermint/tendermint/rpc/test/helpers.go:100 +0x5b
  github.com/tendermint/tendermint/rpc/client_test.TestMain()
      /home/vagrant/go/src/github.com/tendermint/tendermint/rpc/client/main_test.go:17 +0x4c
  main.main()
      github.com/tendermint/tendermint/rpc/client/_test/_testmain.go:76 +0x1cd

Goroutine 47 (running) created at:
  net/http.(*Server).Serve()
      /usr/lib/go-1.9/src/net/http/server.go:2720 +0x37c
  net/http.Serve()
      /usr/lib/go-1.9/src/net/http/server.go:2323 +0xe2
  github.com/tendermint/tendermint/rpc/lib/server.StartHTTPServer.func1()
      /home/vagrant/go/src/github.com/tendermint/tendermint/rpc/lib/server/http_server.go:35 +0xb3
```