export split_partitions/2 from replication_manager.erl

Change-Id: I1aaa6a4e87244f66c2c1f85cb4680989c2621673
Reviewed-on: http://review.couchbase.org/38976
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>