CBD-1039:Purge Expired Items in disk via compactor

With the compactor moved into ep-engine, this commit
adds a new way of purging expired items from disk
When compacting items, we examine if they have expired, if
so, place these items into the appropriate queues to handle
their deletions

Improved unit test for better code coverage

Change-Id: I3b611597a5f0e69ab44237346c2214d67b7fff1c
Reviewed-on: http://review.couchbase.org/29781
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>