fix memory leak in mempool reactor

Leaking goroutine:
```
114 @ 0x42f2bc 0x42f3ae 0x440794 0x4403b9 0x468002 0x9fe32d 0x9ff78f 0xa025ed 0x45e571
```

Explanation:
it blocks on an empty clist forever. so unless theres txs coming in,
this go routine will just sit there, holding onto the peer too.
if we're constantly reconnecting to some peer, old instances are not
garbage collected, leading to memory leak.

Fixes https://github.com/cosmos/gaia/issues/108
Previous attempt https://github.com/tendermint/tendermint/pull/1156

mempool/reactor.go

@@ -2,6 +2,7 @@ package mempool
 
 import (
 	"bytes"
+	"context"
 	"fmt"
 	"reflect"
 	"time"
@@ -101,24 +102,39 @@ type PeerState interface {
 }
 
 // Send new mempool txs to peer.
-// TODO: Handle mempool or reactor shutdown - as is this routine
-// may block forever if no new txs come in.
 func (memR *MempoolReactor) broadcastTxRoutine(peer p2p.Peer) {
 	if !memR.config.Broadcast {
 		return
 	}
 
+	// used to abort waiting until a tx available
+	// otherwise TxsFrontWait/NextWait could block forever if there are
+	// no txs
+	ctx, cancel := context.WithCancel(context.Background())
+	go func() {
+		const healthCheckInterval = 5 * time.Second
+		for {
+			if !memR.IsRunning() || !peer.IsRunning() {
+				cancel()
+				return
+			}
+			time.Sleep(healthCheckInterval)
+		}
+	}()
+
 	var next *clist.CElement
 	for {
-		if !memR.IsRunning() || !peer.IsRunning() {
-			return // Quit!
-		}
+		// This happens because the CElement we were looking at got
+		// garbage collected (removed).  That is, .NextWait() returned nil.
+		// Go ahead and start from the beginning.
 		if next == nil {
-			// This happens because the CElement we were looking at got
-			// garbage collected (removed).  That is, .NextWait() returned nil.
-			// Go ahead and start from the beginning.
-			next = memR.Mempool.TxsFrontWait() // Wait until a tx is available
+			// Wait until a tx is available
+			next = waitWithCancel(memR.Mempool.TxsFrontWait, ctx)
+			if ctx.Err() != nil {
+				return
+			}
 		}
+
 		memTx := next.Value.(*mempoolTx)
 		// make sure the peer is up to date
 		height := memTx.Height()
@@ -136,9 +152,20 @@ func (memR *MempoolReactor) broadcastTxRoutine(peer p2p.Peer) {
 			time.Sleep(peerCatchupSleepIntervalMS * time.Millisecond)
 			continue
 		}
+		next = waitWithCancel(next.NextWait, ctx)
+		if ctx.Err() != nil {
+			return
+		}
+	}
+}
 
-		next = next.NextWait()
-		continue
+func waitWithCancel(f func() *clist.CElement, ctx context.Context) *clist.CElement {
+	el := make(chan *clist.CElement, 1)
+	select {
+	case el <- f():
+		return <-el
+	case <-ctx.Done():
+		return nil
 	}
 }