MB-8066 Check checkpoint persistence completion after flushing.

If there are any checkpoint persistence requests for a given
vbucket, we should check its completion after flushing that
vbucket. Otherwise, it will mostly require another round of a
flusher cycle to check its completion.

Change-Id: Ic6359a89ec2d26d7c8d97d668edffa3ba960c613
Reviewed-on: http://review.couchbase.org/26482
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>