Auto-tuned number of threads should not be less than 4

Some users may run Couchbase Server on machines with 2 cores
(e.g., EC2 instance of type m3.large is still pretty common).
In that case we should use the same numbers of threads as in
previous releases.

Change-Id: Ib13d7ce671010c530321fe9b592d6d2a8ecf7456
Reviewed-on: http://review.couchbase.org/35247
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>