MB-100: added high num of threads for hdgm case

Change-Id: I69d6998c997e09ee42e2b5b8290796b6e3bb4322
Reviewed-on: http://review.couchbase.org/28030
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>