MB-7643: auto-tune number of worker threads
MB-7643: auto-tune number of worker threads

If you have more than 4 cores let's use 75% of them for
frontend threads

Change-Id: I76b3693ff5dfa74532e7d4cabb90a853e6b78c60
Reviewed-on: http://review.couchbase.org/24294
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

cloudrain21/memcached-1,mrkwse/memcached,owendCB/memcached,mrkwse/memcached,mrkwse/memcached,daverigby/memcached,daverigby/memcached,couchbase/memcached,jimwwalker/memcached,mrkwse/memcached,owendCB/memcached,cloudrain21/memcached-1,daverigby/kv_engine,couchbase/memcached,daverigby/kv_engine,daverigby/kv_engine,jimwwalker/memcached,couchbase/memcached,cloudrain21/memcached-1,daverigby/memcached,cloudrain21/memcached-1,jimwwalker/memcached,daverigby/memcached,owendCB/memcached,owendCB/memcached,couchbase/memcached,daverigby/kv_engine