wait_for_flusher_to_settle: Use wait_for_stat_to_be()
wait_for_flusher_to_settle: Use wait_for_stat_to_be()

wait_for_stat_to_be() records the last value (in current) which makes
it easier to debug issues when values arn't as expected. Additionally
this simplifies the code in wait_for_flusher_to_settle.

Change-Id: I601ab4e8b605c937a86b802746cad691ac659308
Reviewed-on: http://review.couchbase.org/69011
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Daniel Owen <b3fc0e1a51f252986e60be7da121b74d3ac6845f@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

daverigby/kv_engine,daverigby/ep-engine,daverigby/ep-engine,membase/ep-engine,couchbase/ep-engine,daverigby/kv_engine,daverigby/kv_engine,couchbase/ep-engine,daverigby/ep-engine,daverigby/ep-engine,couchbase/ep-engine,membase/ep-engine,daverigby/kv_engine,membase/ep-engine,couchbase/ep-engine,membase/ep-engine