MB-8415 change engine parameter num_shards to num_workers

Change-Id: I5035442233e1208e7a9a7540382a38ba4dec810a
Reviewed-on: http://review.couchbase.org/26776
Tested-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>