Revert "MB-8415 change engine parameter num_shards to num_workers"

This reverts commit c84d26d7e27f63f5ad6919d18d8392cd590dd983.

Change-Id: Ic17b36277106f7de77a351e0ce8d8dc45e675e02
Reviewed-on: http://review.couchbase.org/26786
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>