MB-11725: Place rollback task on the writer dispatcher

The rollback task writes a header on success and as a result we need
to make sure that this doesn't race with other writer tasks. We also
need to keep in mind that the cached vbucket states in couch kvstore
are only valid for the rw kvstore.

Change-Id: Ib1edefb50817ac8240165d93d35e5dc89f4db042
Reviewed-on: http://review.couchbase.org/40017
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>