MB-100 Really set default number of readers/writers to 3.

Change-Id: I64d014ead6a90a706a6ff030023115eecf7250c0
Reviewed-on: http://review.couchbase.org/26871
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>