remove comment about LRU cache (see comments below)

I've tried https://github.com/hashicorp/golang-lru/tree/master/simplelru today and here are
the results:

with LRU cache:

```
Benchmark10Clients-2                               50000             29021 ns/op         3976 B/op         105 allocs/op
Benchmark100Clients-2                               3000            363432 ns/op        36382 B/op        1005 allocs/op
Benchmark1000Clients-2                               500           2473752 ns/op       360500 B/op       10009 allocs/op
Benchmark10ClientsUsingTheSameQuery-2             300000              4059 ns/op          773 B/op          15 allocs/op
Benchmark100ClientsUsingTheSameQuery-2            500000              4360 ns/op          773 B/op          15 allocs/op
Benchmark1000ClientsUsingTheSameQuery-2           300000              4204 ns/op          773 B/op          15 allocs/op
```

without LRU cache:

```
Benchmark10Clients-2                      200000              5267 ns/op             616 B/op       25 allocs/op
Benchmark100Clients-2                      30000             42134 ns/op            2776 B/op      205 allocs/op
Benchmark1000Clients-2                      3000            552648 ns/op           24376 B/op     2005 allocs/op
Benchmark10ClientsOneQuery-2             1000000              2127 ns/op             462 B/op        9 allocs/op
Benchmark100ClientsOneQuery-2             500000              2353 ns/op             462 B/op        9 allocs/op
Benchmark1000ClientsOneQuery-2            500000              2339 ns/op             462 B/op        9 allocs/op
```

> How were you using the lru cache exactly?

I was adding a KV pair each time there is a match plus checking if
`lru.Contains(key)` before running the actual check (`q.Matches(tags)`).

```
key = fmt.Sprintf("%s/%v", query + tags)
```

pubsub/pubsub.go

@@ -290,11 +290,8 @@ func (state *state) removeAll(clientID string) {
 
 func (state *state) send(msg interface{}, tags map[string]interface{}, slowClientStrategy overflowStrategy, logger log.Logger) {
 	for q, clientToChannelMap := range state.queries {
-		// NOTE we can use LRU cache to speed up common cases like query = "
-		// tm.events.type=NewBlock" and tags = {"tm.events.type": "NewBlock"}
 		if q.Matches(tags) {
 			for clientID, ch := range clientToChannelMap {
-				logger.Info("Sending message to client", "msg", msg, "client", clientID)
 				switch slowClientStrategy {
 				case drop:
 					select {

pubsub/pubsub_test.go

@@ -193,13 +193,21 @@ func Benchmark10Clients(b *testing.B)   { benchmarkNClients(10, b) }
 func Benchmark100Clients(b *testing.B)  { benchmarkNClients(100, b) }
 func Benchmark1000Clients(b *testing.B) { benchmarkNClients(1000, b) }
 
+func Benchmark10ClientsOneQuery(b *testing.B)   { benchmarkNClientsOneQuery(10, b) }
+func Benchmark100ClientsOneQuery(b *testing.B)  { benchmarkNClientsOneQuery(100, b) }
+func Benchmark1000ClientsOneQuery(b *testing.B) { benchmarkNClientsOneQuery(1000, b) }
+
 func benchmarkNClients(n int, b *testing.B) {
-	s := pubsub.NewServer(pubsub.BufferCapacity(b.N))
+	s := pubsub.NewServer()
 	s.Start()
 	defer s.Stop()
 
 	for i := 0; i < n; i++ {
 		ch := make(chan interface{})
+		go func() {
+			for range ch {
+			}
+		}()
 		s.Subscribe(clientID, query.MustParse(fmt.Sprintf("abci.Account.Owner = Ivan AND abci.Invoices.Number = %d", i)), ch)
 	}
 
@@ -210,6 +218,28 @@ func benchmarkNClients(n int, b *testing.B) {
 	}
 }
 
+func benchmarkNClientsOneQuery(n int, b *testing.B) {
+	s := pubsub.NewServer()
+	s.Start()
+	defer s.Stop()
+
+	q := query.MustParse("abci.Account.Owner = Ivan AND abci.Invoices.Number = 1")
+	for i := 0; i < n; i++ {
+		ch := make(chan interface{})
+		go func() {
+			for range ch {
+			}
+		}()
+		s.Subscribe(clientID, q, ch)
+	}
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		s.PublishWithTags("Gamora", map[string]interface{}{"abci.Account.Owner": "Ivan", "abci.Invoices.Number": 1})
+	}
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 /// HELPERS
 ///////////////////////////////////////////////////////////////////////////////