MB-8415 change num_shards parameter to num_workers

The parameter is really for setting an approximate
total number of worker threads (num_workers) not
a total number of shards for given bucket (num_shards).

Change-Id: Ica52b0b03f4da4106f46a0a4bee3bee4957f9e01
Reviewed-on: http://review.couchbase.org/26772
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Jin Lim <jin@couchbase.com>