MB-7543 Rev sequence number shouldn't be set to zero.

We should make sure that the revision sequence number shouldn't
be zero during warmup or when we receive tap_deletion,
tap_mutation, set_with_meta, and delete_with_meta.

Change-Id: I4316e0b688e2f29efc94533fab6fe61df0ea871a
Reviewed-on: http://review.couchbase.org/24004
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Reviewed-on: http://review.couchbase.org/24007
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Free persistence calbacks in CouchKVStore

Change-Id: Ic3735c1712341860da792660262dfe2d1366f009
WIP: Rewrite the flusher logic

Change-Id: I39e9c25ad3380b3e045002967c9532b16758e187
WIP

Change-Id: I07b99ea33c8e03fa67de52e8c840c3d373fc6a76
WIP

Change-Id: Iae4e7fc2988adf8fd7290d6ee86b3ddb74faf9c8