mempool: add txs from Update to cache

We should add txs that come in from mempool.Update to the mempool's
cache, so that they never hit a potentially expensive check tx.

Originally posted by @ValarDragon in #2846
https://github.com/tendermint/tendermint/issues/2846#issuecomment-439216656

Refs #2855

mempool/mempool.go

@@ -545,6 +545,11 @@ func (mem *Mempool) Update(
 		mem.postCheck = postCheck
 	}
 
+	// Add committed transactions to cache (if missing).
+	for _, tx := range txs {
+		_ = mem.cache.Push(tx)
+	}
+
 	// Remove committed transactions.
 	txsLeft := mem.removeTxs(txs)
 

mempool/mempool_test.go

@@ -163,6 +163,17 @@ func TestMempoolFilters(t *testing.T) {
 	}
 }
 
+func TestMempoolUpdateAddsTxsToCache(t *testing.T) {
+	app := kvstore.NewKVStoreApplication()
+	cc := proxy.NewLocalClientCreator(app)
+	mempool := newMempoolWithApp(cc)
+	mempool.Update(1, []types.Tx{[]byte{0x01}}, nil, nil)
+	err := mempool.CheckTx([]byte{0x01}, nil)
+	if assert.Error(t, err) {
+		assert.Equal(t, ErrTxInCache, err)
+	}
+}
+
 func TestTxsAvailable(t *testing.T) {
 	app := kvstore.NewKVStoreApplication()
 	cc := proxy.NewLocalClientCreator(app)