MB-8214 allocate writers equal to half of total readers

Change-Id: Ia15935e8e8f753fb914125ead2800fa04a8b9570
Reviewed-on: http://review.couchbase.org/26394
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Jin Lim <jin@couchbase.com>