MB-100: increase the number of default fill rates

The header file defines the default values for minimum and
maximum fill rate. Increase it from min 1 to 25 and from
max 5 to 40.

Change-Id: I3545c37af8e2bfda61dd5a0e20acce1c7a5b427b
Reviewed-on: http://review.couchbase.org/25133
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>