MINOR: stream: rename {srv,prx}_queue_size to *_queue_pos

MINOR: stream: rename {srv,prx}_queue_size to *_queue_pos

The current name is misleading as it implies a queue size, but the value
instead indicates a position in the queue.
The value is only the queue size at the exact moment the element is enqueued.
Soon we will gain the ability to insert anywhere into the queue, upon which
clarity of the name is more important.