[libs/pubsub] fix memory leak

Refs #1755

I started with writing a test for wsConnection (WebsocketManager) where
I:

- create a WS connection
- do a simple echo call
- close it

No leaking goroutines, nor any leaking memory were detected.

For useful shortcuts see my blog post
https://blog.cosmos.network/debugging-the-memory-leak-in-tendermint-210186711420

Then I went to the rpc tests to see if calling Subscribe results in
memory growth. It did.

I used a slightly modified version of TestHeaderEvents function:

```
func TestHeaderEvents(t *testing.T) {
	// memory heap before
	f, err := os.Create("/tmp/mem1.mprof")
	if err != nil {
		t.Fatal(err)
	}
	pprof.WriteHeapProfile(f)
	f.Close()

	for i := 0; i < 100; i++ {
		c := getHTTPClient()
		err = c.Start()
		require.Nil(t, err)

		evtTyp := types.EventNewBlockHeader
		evt, err := client.WaitForOneEvent(c, evtTyp, waitForEventTimeout)
		require.Nil(t, err)
		_, ok := evt.(types.EventDataNewBlockHeader)
		require.True(t, ok)

		c.Stop()
		c = nil
	}

	runtime.GC()

	// memory heap before
	f, err = os.Create("/tmp/mem2.mprof")
	if err != nil {
		t.Fatal(err)
	}
	pprof.WriteHeapProfile(f)
	f.Close()

	// dump all running goroutines
	time.Sleep(10 * time.Second)
	pprof.Lookup("goroutine").WriteTo(os.Stdout, 1)
}
```

```
Showing nodes accounting for 35159.16kB, 100% of 35159.16kB total
Showing top 10 nodes out of 48
      flat  flat%   sum%        cum   cum%
32022.23kB 91.08% 91.08% 32022.23kB 91.08%  github.com/tendermint/tendermint/libs/pubsub/query.(*QueryParser).Init
 1056.33kB  3.00% 94.08%  1056.33kB  3.00%  bufio.NewReaderSize
  528.17kB  1.50% 95.58%   528.17kB  1.50%  bufio.NewWriterSize
  528.17kB  1.50% 97.09%   528.17kB  1.50%  github.com/tendermint/tendermint/consensus.NewConsensusState
  512.19kB  1.46% 98.54%   512.19kB  1.46%  runtime.malg
  512.08kB  1.46%   100%   512.08kB  1.46%  syscall.ByteSliceFromString
         0     0%   100%   512.08kB  1.46%  github.com/tendermint/tendermint/consensus.(*ConsensusState).(github.com/tendermint/tendermint/consensus.defaultDecideProposal)-fm
         0     0%   100%   512.08kB  1.46%  github.com/tendermint/tendermint/consensus.(*ConsensusState).addVote
         0     0%   100%   512.08kB  1.46%  github.com/tendermint/tendermint/consensus.(*ConsensusState).defaultDecideProposal
         0     0%   100%   512.08kB  1.46%  github.com/tendermint/tendermint/consensus.(*ConsensusState).enterNewRound
```

100 subscriptions produce 32MB.

Again, no additional goroutines are running after the end of the test
(wsConnection readRoutine and writeRoutine both finishes). **It means
that some exiting goroutine or object is holding a reference to the
*Query objects, which are leaking.**

One of them is pubsub#loop. It's using state.queries to map queries to
clients and state.clients to map clients to queries.

Before this commit, we're not thoroughly cleaning state.queries, which
was the reason for memory leakage.

libs/pubsub/pubsub.go

@@ -156,6 +156,8 @@ func (s *Server) Subscribe(ctx context.Context, clientID string, query Query, ou
 		if _, ok = s.subscriptions[clientID]; !ok {
 			s.subscriptions[clientID] = make(map[string]Query)
 		}
+		// preserve original query
+		// see Unsubscribe
 		s.subscriptions[clientID][query.String()] = query
 		s.mtx.Unlock()
 		return nil
@@ -314,6 +316,9 @@ func (state *state) remove(clientID string, q Query) {
 		}
 
 		delete(state.queries[q], clientID)
+		if len(state.queries[q]) == 0 {
+			delete(state.queries, q)
+		}
 	}
 }
 
@@ -328,8 +333,10 @@ func (state *state) removeAll(clientID string) {
 		close(ch)
 
 		delete(state.queries[q], clientID)
+		if len(state.queries[q]) == 0 {
+			delete(state.queries, q)
+		}
 	}
-
 	delete(state.clients, clientID)
 }