MB-8657 fix total number of threads allocated for MRW

Upon this fix the total number of threads allocated for MRW
workers matches the max thread number specified by users via
UI.

Certain setting such as 4, 5, 7 may break the workload ratio (2:1)
btw numbers of readers and writers but will conform to the user
input thus avoid any confusion.

Change-Id: I3e45ebc38903d37df6746a9f20b9a1e8f6919e03
Reviewed-on: http://review.couchbase.org/28096
Tested-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>