CBQE-1944: fix compaction task

Exit task if fragmentation is 0.
Monitor fragmentation,if new_fragmentation is less than old_fragmentation consider compaction was run

Change-Id: Ifd304c52dbe3cb3a4f62a629531d4db81e682fdd
Reviewed-on: http://review.couchbase.org/33029
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>