MB-100: feed mrw thread settings from conf files

Change-Id: I8024764a075875964a868d6ceb56e48b41ba4ddd
Reviewed-on: http://review.couchbase.org/27508
Reviewed-by: abhinav dangeti <abhinav.dangeti@gmail.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>