MB-11731 compact vbuckets in parallel

...because it speeds up the compaction

number of workes is configurable via key: compaction_number_of_kv_workers
default is 4

Change-Id: I83b76f0b88652ac702b41e4dbaa6eb0654cd669b
Reviewed-on: http://review.couchbase.org/39647
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>