MINOR: servers: Introduce src_manage_queues()

MINOR: servers: Introduce src_manage_queues()

Instead of having a common pattern of
	if (may_dequeue_tasks())
		process_srv_queue()
introduce an inline function to do that, and use it where appropriate
(mostly everywhere where the pattern was used).
Later on this function will be responsible for figuring out if, after
the queue has been processed, the server is full or not.