Rapidly cancelling a Call can overwhelm transport

Doing something like

```
ClientCall call = channel.newCall(foo, CallOptions.DEFAULT);
call.start(listen, new Metadata());
call.sendMessage(null);
call.halfClose();
call.request(1);
call.cancel();
```

rapidly in a loop causes memory to not get reclaimed. Somehow netty is holding on to commands sent to it from the ClientCallImpl.