This commit implements a policy of writing as many queued buffers as the
writable socket allows, until non-writable, or the write queue is empty.
This has two objectives: 1) any allocated buffers for data copies will
be freed sooner, 2) the write output will be more responsive, fixing at
least one known delay in WiFi scanlist output in YUBOX Now.
This prevents a race condicion of the ::add() method modifying the queue
and being preempted by the high-priority asyncTcpSock task that in turn
flushes the same queue to the socket, therefore modifying the queue.
