MB-100: multithread analysis for read heavy load

Change-Id: I6a9e68dae7b59ad13251d13cdba65e40c1b24802
Reviewed-on: http://review.couchbase.org/28242
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>