MB-9464: add stat ep_num_workers for thread count

Change-Id: I1a0137e6ee1a402364d417adaf2cff9f7dcd4acc
Reviewed-on: http://review.couchbase.org/30679
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>