MB-2512 - allow replica count to be edited

if the replica count is changed the janitor will
adjust the size of the chain in the buckets map

Change-Id: I8597f603d6dd4e2affcdfc4d47abd214ea18b373
Reviewed-on: http://review.couchbase.org/28854
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>