MB-8657 Fix the number of workers to be matched for mix loads

This change fixes the issue for a case where the number of
actual worker threads is not matched with the number of readers
and writers given by the user.

Change-Id: I1c6b1284bd4677af715603d01d1829b6611a87f0
Reviewed-on: http://review.couchbase.org/28325
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>