MB-100: added eight thread tuning

Change-Id: I2b419d3c9ae0e11809fea193fd6f18c9afd8c076
Reviewed-on: http://review.couchbase.org/27527
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>