Set processed queue items to `null` to allow garbage collection of these
items. The command queue contains callbacks. They contain the stack of
the current fiber, which can be large. Allow earlier garbage collection
of these stacks by removing the reference to the queue items.
Simply and speed up command argument processing logic.
Commands now return the true/false value from the underlying socket write(s).
Implement command_queue high water and low water for more better control of queueing.
* connection error did not properly trigger reconnection logic [GH-85]
* client.hmget(key, [val1, val2]) was not expanding properly [GH-66]
* client.quit() while in pub/sub mode would throw an error [GH-87]
* client.multi(['hmset', 'key', {foo: 'bar'}]) fails [GH-92]
