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 <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>