CBQE-0: flush in-memory queue every 2 mins

the memq is for fast key access and is supposed to be
occasionaly flused to disk after certain # of items added.
however if it grows too large it is 'ALWAYS' flushed,
so setting it instead to flush every 120 cycles

Change-Id: I041d97aa3f0d59b557b68bbae54427f3253655d4
Reviewed-on: http://review.couchbase.org/30460
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>