MB-6368: Move warmup logic out of kvstore

Change-Id: Ida71ec248eba7c169009717a3ac9d2aeef9c32a4
Reviewed-on: http://review.couchbase.org/24800
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>