MB-8231 flusher must continue checkpoint persistence for empty vbucket

This fixes the current doHighPriority logic that incorrectly assumes
there must be always pending mutation (low priority vbucket
persistence) when more than one vbucket is marked for checkpoint
persistence. Otherwise, this could cause the rebalance to hang when
ns server issues the checkpoint_persistence cmd over empty vbucket or
vbuckets being created.

Change-Id: I4d23d1d49c8f6e9c23041afbf65137312e77ce2f
Reviewed-on: http://review.couchbase.org/26275
Tested-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Jin Lim <jin@couchbase.com>