fix TestFullRound1 race (Refs #846)

```
==================
WARNING: DATA RACE
Write at 0x00c42d7605f0 by goroutine 844:
  github.com/tendermint/tendermint/consensus.(*ConsensusState).updateToState()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:465 +0x59e
I[11-14|22:37:28.781] Added to prevote                             vote="Vote{0:646753DCE124 1/02/1(Prevote) E9B19636DCDB {/CAD5FA805E8C.../}}" prevotes="VoteSet{H:1 R:2 T:1 +2/3:<nil> BA{2:X_} map[]}"
  github.com/tendermint/tendermint/consensus.(*ConsensusState).finalizeCommit()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:1229 +0x16a9
  github.com/tendermint/tendermint/consensus.(*ConsensusState).tryFinalizeCommit()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:1135 +0x721
  github.com/tendermint/tendermint/consensus.(*ConsensusState).enterCommit.func1()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:1087 +0x153
  github.com/tendermint/tendermint/consensus.(*ConsensusState).enterCommit()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:1114 +0xa34
  github.com/tendermint/tendermint/consensus.(*ConsensusState).addVote()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:1423 +0xdd6
  github.com/tendermint/tendermint/consensus.(*ConsensusState).tryAddVote()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:1317 +0x77
  github.com/tendermint/tendermint/consensus.(*ConsensusState).handleMsg()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:565 +0x7a9
  github.com/tendermint/tendermint/consensus.(*ConsensusState).receiveRoutine()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:523 +0x6d2

Previous read at 0x00c42d7605f0 by goroutine 654:
  github.com/tendermint/tendermint/consensus.validatePrevote()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/common_test.go:149 +0x57
  github.com/tendermint/tendermint/consensus.TestFullRound1()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state_test.go:256 +0x3c5
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:746 +0x16c

Goroutine 844 (running) created at:
  github.com/tendermint/tendermint/consensus.(*ConsensusState).startRoutines()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state.go:258 +0x8c
  github.com/tendermint/tendermint/consensus.startTestRound()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/common_test.go:118 +0x63
  github.com/tendermint/tendermint/consensus.TestFullRound1()
      /home/vagrant/go/src/github.com/tendermint/tendermint/consensus/state_test.go:247 +0x1fb
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:746 +0x16c

Goroutine 654 (running) created at:
  testing.(*T).Run()
      /usr/local/go/src/testing/testing.go:789 +0x568
  testing.runTests.func1()
      /usr/local/go/src/testing/testing.go:1004 +0xa7
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:746 +0x16c
  testing.runTests()
      /usr/local/go/src/testing/testing.go:1002 +0x521
  testing.(*M).Run()
      /usr/local/go/src/testing/testing.go:921 +0x206
  main.main()
      github.com/tendermint/tendermint/consensus/_test/_testmain.go:106 +0x1d3
==================
```

consensus/state_test.go

@@ -10,6 +10,7 @@ import (
 	cstypes "github.com/tendermint/tendermint/consensus/types"
 	"github.com/tendermint/tendermint/types"
 	cmn "github.com/tendermint/tmlibs/common"
+	"github.com/tendermint/tmlibs/log"
 	tmpubsub "github.com/tendermint/tmlibs/pubsub"
 )
 
@@ -240,6 +241,14 @@ func TestFullRound1(t *testing.T) {
 	cs, vss := randConsensusState(1)
 	height, round := cs.Height, cs.Round
 
+	// NOTE: buffer capacity of 0 ensures we can validate prevote and last commit
+	// before consensus can move to the next height (and cause a race condition)
+	cs.eventBus.Stop()
+	eventBus := types.NewEventBusWithBufferCapacity(0)
+	eventBus.SetLogger(log.TestingLogger().With("module", "events"))
+	cs.SetEventBus(eventBus)
+	eventBus.Start()
+
 	voteCh := subscribe(cs.eventBus, types.EventQueryVote)
 	propCh := subscribe(cs.eventBus, types.EventQueryCompleteProposal)
 	newRoundCh := subscribe(cs.eventBus, types.EventQueryNewRound)