Changes

• Removed unused import of testify.
• In newManagedConn(conn *Conn, key KeyType) i was setting conn.onClose without protecting it with the mutex. So i added lock/unlock of the onCloseMu.
• Added OnJoin/OnLeave hooks for rooms, and DefaultOnJoin/Leave for the room manager.
• In the room struct the "key" field was private, but it was meant to be public. So now it is public.