Send stats through gen_server

The stats should be send through the gen_server, so that they have
the same flow as they would be send from ep-engine.

Change-Id: Ife6eff2634947083149f59b9941c5abb34a11013
Reviewed-on: http://review.couchbase.org/34435
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>