MB-8066 Interleave flushing high-priority vbs with low-priority vbs

This change makes sure that flushing high-priority vbuckets
interleaves with lower-priority vbuckets.

Change-Id: I9d1eef904ec34d06b018cb113c5943120821fd89
Reviewed-on: http://review.couchbase.org/26438
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Jin Lim <jin@couchbase.com>