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_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 	"time"
 
+	"github.com/fortytw2/leaktest"
+	"github.com/pkg/errors"
 	"github.com/stretchr/testify/assert"
 
 	"github.com/go-kit/kit/log/term"
@@ -91,18 +93,65 @@ func _waitForTxs(t *testing.T, wg *sync.WaitGroup, txs types.Txs, reactorIdx int
 	wg.Done()
 }
 
-var (
+const (
 	NUM_TXS = 1000
 	TIMEOUT = 120 * time.Second // ridiculously high because CircleCI is slow
 )
 
 func TestReactorBroadcastTxMessage(t *testing.T) {
 	config := cfg.TestConfig()
-	N := 4
+	const N = 4
 	reactors := makeAndConnectMempoolReactors(config, N)
+	defer func() {
+		for _, r := range reactors {
+			r.Stop()
+		}
+	}()
 
 	// send a bunch of txs to the first reactor's mempool
 	// and wait for them all to be received in the others
 	txs := checkTxs(t, reactors[0].Mempool, NUM_TXS)
 	waitForTxs(t, txs, reactors)
 }
+
+func TestBroadcastTxForPeerStopsWhenPeerStops(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode.")
+	}
+
+	config := cfg.TestConfig()
+	const N = 2
+	reactors := makeAndConnectMempoolReactors(config, N)
+	defer func() {
+		for _, r := range reactors {
+			r.Stop()
+		}
+	}()
+
+	// stop peer
+	sw := reactors[1].Switch
+	sw.StopPeerForError(sw.Peers().List()[0], errors.New("some reason"))
+
+	// check that we are not leaking any go-routines
+	// i.e. broadcastTxRoutine finishes when peer is stopped
+	leaktest.CheckTimeout(t, 10*time.Second)()
+}
+
+func TestBroadcastTxForPeerStopsWhenReactorStops(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode.")
+	}
+
+	config := cfg.TestConfig()
+	const N = 2
+	reactors := makeAndConnectMempoolReactors(config, N)
+
+	// stop reactors
+	for _, r := range reactors {
+		r.Stop()
+	}
+
+	// check that we are not leaking any go-routines
+	// i.e. broadcastTxRoutine finishes when reactor is stopped
+	leaktest.CheckTimeout(t, 10*time.Second)()
+}