Defragmenter performance test

Create a new ep-engine unit test to measure the speed of the
defragmenter.

Change-Id: I809a24906596ee521402f41c68605531a3301e8e
Reviewed-on: http://review.couchbase.org/44286
Reviewed-by: Jim Walker <jim@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Dave Rigby <daver@couchbase.com>