Plat 592 writable sources

Well, the messages are queued. The bool result of "write" indicates whether there is room remaining in the queue. For fixed size queues, we can either block, throw or return "false". I prefer the latter. Regarding "queueEnabled", I can change its name, but its purpose is to indicate whether the connection has been open or is about to be open. For example, tcp sockets enables non-blocking connections and in this case, I think it is acceptable to enable the "write" method, even though the connection has not been fully estabished yet.