MB-100 ns_memcached:get_vbucket_details_stats

will be used in compaction_daemon for obtaining individual
vbucket DiskSize and DataSize

Change-Id: Ib6c0639525b3c4cbcd6dce6da3934a7006efe299
Reviewed-on: http://review.couchbase.org/32827
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>