MB-36301: Prefer O(n) expelled list iterations
MB-36301: Prefer O(n) expelled list iterations

This commit is no way a fix for any observed performance issue,
but during the bulk of reverts for MB-36301 it was noted that
expelItems performs O(2n) iterations of the expelled items for
open checkpoints.

Change-Id: I03828b8883fdaed16758da4bf4d443d984911732
Reviewed-on: http://review.couchbase.org/116075
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine