MB-3031 - Schedule item pager tasks by the non-IO dispatcher
MB-3031 - Schedule item pager tasks by the non-IO dispatcher

As item pager tasks don't perform any disk IO operations through
the sqlite, they can be scheduled by the non-IO dispatcher.
This also resolves the starvation issue on item pager tasks in the
EC2, in case that the flusher (higher priority than item pager) is
too busy with the large persistence queue.

Change-Id: I04b92708cc5ce1326a3150c2fc3bd0c6ea605a5d
Reviewed-on: http://review.membase.org/3916
Tested-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>
Reviewed-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>

owendCB/ep-engine,daverigby/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,zbase/ep-engine,membase/ep-engine,owendCB/ep-engine,couchbase/ep-engine,daverigby/ep-engine,couchbaselabs/ep-engine,couchbaselabs/ep-engine,jimwwalker/ep-engine,abhinavdangeti/ep-engine,abhinavdangeti/ep-engine,sriganes/ep-engine,membase/ep-engine,jimwwalker/ep-engine,abhinavdangeti/ep-engine,daverigby/ep-engine,owendCB/ep-engine,zbase/ep-engine,hisundar/ep-engine,zbase/ep-engine,zbase/ep-engine,couchbase/ep-engine,jimwwalker/ep-engine,zbase/ep-engine,couchbase/ep-engine,owendCB/ep-engine,couchbaselabs/ep-engine,daverigby/kv_engine,daverigby/ep-engine,teligent-ru/ep-engine,teligent-ru/ep-engine,membase/ep-engine,daverigby/kv_engine,abhinavdangeti/ep-engine,membase/ep-engine,couchbase/ep-engine,couchbaselabs/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,sriganes/ep-engine,abhinavdangeti/ep-engine,jimwwalker/ep-engine,sriganes/ep-engine,sriganes/ep-engine,daverigby/kv_engine,daverigby/kv_engine,hisundar/ep-engine,couchbaselabs/ep-engine