CBD-485: Move DB file compaction into EP-Engine

As detailed in CBD-485, the compactor module is moved into EP-Engine
New Memcached Command:
+ CMD_COMPACT_DB with protocol_binary_request_compact_db
New Stats:
+ ep_pending_compactions
+ compaction timing histogram
New unit test case to exercise the code path
New command line utility to perform compaction:
+ cbcompact 127.0.0.1:12000 compact <vbid> --wait
New MCCouch Notification cmd
+ VB_COMPACTION_DONE VB_COMPACT_OPENDB_ERROR VB_COMPACT_RENAME_ERROR
Change-Id: I0606a6e06edc41c5265f3661d97d178165c7f19a
Reviewed-on: http://review.couchbase.org/29258
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>