GR = goroutine
[GR#1] http server gets closed
[GR#2] client.NotifyClose() will be executed
[GR#2] client.once.Do will be executed (lock's client.once.m)
[GR#1] stream.Close will be executed (lock's stream.lock)
[GR#1] client.Close will be executed (waits for client.once.m)
[GR#2] stream.remove will be executed (waits for stream.lock)
GR#1 holds lock stream.lock and waits for client.once.m
GR#2 holds lock client.once.m and waits for stream.lock
We prevent the deadlock with releasing the client.once.m lock earlier.
