Allow to control implicit flush behavior

Change-Id: I42b0c301f0a94e187c0daa2520046dced37b030c
Reviewed-on: http://review.couchbase.org/41611
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>