MB-8415 set minum num_threads back to 2 and set default to 3

Change-Id: Ib82f15f31155dd8f9e4f37073b27115f300549d2
Reviewed-on: http://review.couchbase.org/26796
Reviewed-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Jin Lim <jin@couchbase.com>