MB-9800 Always respond to CHECKPOINT_PERSISTENCE through flusher

Even if a requested checkpoint is already persisted, let the
flusher respond to CHECKPOINT_PERSISTENCE command to prevent any
race conditions between the database storage engine and
view indexer.

Change-Id: I26a4d77d97c4c1d93957621f746e09bee2e2ebb4
Reviewed-on: http://review.couchbase.org/32610
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>