move stats into a trigger

[marcopeg]

IF NEW.status <> OLD.status THEN change counters

this way we can activate/deactivate counters by simply attaching or detaching the trigger.

heavily used queues can probably do without triggers and we might simply provide a way to estimate pending (reading through the index??)

normal queues can also disable the trigger while bulk inserting and reset the metrics when insert ends.

http://www.postgresqltutorial.com/creating-first-trigger-postgresql/

fetchq_queue_enable_metrics(queue, true/false)