Defragmenter: Use ExTask RCPTR

Use a ExTask (reference-counted pointer) for the defragmenterTask to
ensure that there isn't any races between the ExecutorPool using it
and shutting the task down.

Fixes some intermittent unit-test failures seen on OS X.

Change-Id: Icc7bbea8bbdd3fb26c8183106a2568dcb95275d5
Reviewed-on: http://review.couchbase.org/43243
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>