MB-12067 [2/6]: Expose run_defragmenter_task from ep_engine via epctl

Add support en/disabling defragmentation, and explicitly triggering
defragmentation via the epctl interface. The main use-case for this is
to allow unit-tests to explicitly trigger the defragmenter.

Change-Id: Ieaeadfbd4186213e35a1e89bc0202d39ff27ab96
Reviewed-on: http://review.couchbase.org/41495
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Dave Rigby <daver@couchbase.com>