CBQE-2136: add a bucket compaction task

adding a task for manually triggering compaction. this is useful for purging non-resident items.

Change-Id: Ia87c03c98a8baa06d08b8fa372d1ea24c09cfa96
Reviewed-on: http://review.couchbase.org/34790
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>