protect Record* peerStateStats functions by mutex

Fixes #1414

DATA RACE:
```
Read at 0x00c4214ee940 by goroutine 146:
  github.com/tendermint/tendermint/consensus.(*peerStateStats).String()
      <autogenerated>:1 +0x57
  fmt.(*pp).handleMethods()
      /usr/local/go/src/fmt/print.go:596 +0x3f4
  fmt.(*pp).printArg()
      /usr/local/go/src/fmt/print.go:679 +0x11f
  fmt.(*pp).doPrintf()
      /usr/local/go/src/fmt/print.go:996 +0x319
  fmt.Sprintf()
      /usr/local/go/src/fmt/print.go:196 +0x73
  github.com/tendermint/tendermint/consensus.(*PeerState).StringIndented()
      github.com/tendermint/tendermint/consensus/_test/_obj_test/reactor.go:1426 +0x573
  github.com/tendermint/tendermint/consensus.(*PeerState).String()
      github.com/tendermint/tendermint/consensus/_test/_obj_test/reactor.go:1419 +0x66
  github.com/go-logfmt/logfmt.safeString()
      /home/ubuntu/go/src/github.com/go-logfmt/logfmt/encode.go:299 +0x9d
  github.com/go-logfmt/logfmt.writeValue()
      /home/ubuntu/go/src/github.com/go-logfmt/logfmt/encode.go:217 +0x5a0
  github.com/go-logfmt/logfmt.(*Encoder).EncodeKeyval()
      /home/ubuntu/go/src/github.com/go-logfmt/logfmt/encode.go:61 +0x1dd
  github.com/tendermint/tmlibs/log.tmfmtLogger.Log()
      /home/ubuntu/go/src/github.com/tendermint/tmlibs/log/tmfmt_logger.go:107 +0x1001
  github.com/tendermint/tmlibs/log.(*tmfmtLogger).Log()
      <autogenerated>:1 +0x93
  github.com/go-kit/kit/log.(*context).Log()
      /home/ubuntu/go/src/github.com/go-kit/kit/log/log.go:124 +0x248
  github.com/tendermint/tmlibs/log.(*tmLogger).Debug()
      /home/ubuntu/go/src/github.com/tendermint/tmlibs/log/tm_logger.go:64 +0x1d0
  github.com/tendermint/tendermint/consensus.(*PeerState).PickSendVote()
      github.com/tendermint/tendermint/consensus/_test/_obj_test/reactor.go:1059 +0x242
  github.com/tendermint/tendermint/consensus.(*ConsensusReactor).gossipVotesForHeight()
      github.com/tendermint/tendermint/consensus/_test/_obj_test/reactor.go:789 +0x6ef
  github.com/tendermint/tendermint/consensus.(*ConsensusReactor).gossipVotesRoutine()
      github.com/tendermint/tendermint/consensus/_test/_obj_test/reactor.go:723 +0x1039

Previous write at 0x00c4214ee940 by goroutine 21:
  github.com/tendermint/tendermint/consensus.(*PeerState).RecordVote()
      github.com/tendermint/tendermint/consensus/_test/_obj_test/reactor.go:1242 +0x15a
  github.com/tendermint/tendermint/consensus.(*ConsensusReactor).Receive()
      github.com/tendermint/tendermint/consensus/_test/_obj_test/reactor.go:309 +0x32e6
  github.com/tendermint/tendermint/p2p.createMConnection.func1()
      /home/ubuntu/go/src/github.com/tendermint/tendermint/p2p/peer.go:365 +0xea
  github.com/tendermint/tendermint/p2p/conn.(*MConnection).recvRoutine()
      /home/ubuntu/go/src/github.com/tendermint/tendermint/p2p/conn/connection.go:531 +0x779
```

consensus/reactor.go

@@ -1087,6 +1087,9 @@ func (ps *PeerState) ensureVoteBitArrays(height int64, numValidators int) {
 // It returns the total number of votes (1 per block). This essentially means
 // the number of blocks for which peer has been sending us votes.
 func (ps *PeerState) RecordVote(vote *types.Vote) int {
+	ps.mtx.Lock()
+	defer ps.mtx.Unlock()
+
 	if ps.stats.lastVoteHeight >= vote.Height {
 		return ps.stats.votes
 	}
@@ -1098,13 +1101,20 @@ func (ps *PeerState) RecordVote(vote *types.Vote) int {
 // VotesSent returns the number of blocks for which peer has been sending us
 // votes.
 func (ps *PeerState) VotesSent() int {
+	ps.mtx.Lock()
+	defer ps.mtx.Unlock()
+
 	return ps.stats.votes
 }
 
-// RecordVote updates internal statistics for this peer by recording the block part.
-// It returns the total number of block parts (1 per block). This essentially means
-// the number of blocks for which peer has been sending us block parts.
+// RecordBlockPart updates internal statistics for this peer by recording the
+// block part. It returns the total number of block parts (1 per block). This
+// essentially means the number of blocks for which peer has been sending us
+// block parts.
 func (ps *PeerState) RecordBlockPart(bp *BlockPartMessage) int {
+	ps.mtx.Lock()
+	defer ps.mtx.Unlock()
+
 	if ps.stats.lastBlockPartHeight >= bp.Height {
 		return ps.stats.blockParts
 	}
@@ -1117,6 +1127,9 @@ func (ps *PeerState) RecordBlockPart(bp *BlockPartMessage) int {
 // BlockPartsSent returns the number of blocks for which peer has been sending
 // us block parts.
 func (ps *PeerState) BlockPartsSent() int {
+	ps.mtx.Lock()
+	defer ps.mtx.Unlock()
+
 	return ps.stats.blockParts
 }