Add support for 0-based partition IDs

Initial goal is to allow user commands to specify partition IDs as short integers `[0, ring_size)` instead of hash values. May consolidate other features such as Yokozuna's logical partitions later.
An alternative to number one would be to not support passing in the ring here. Only the ring size. Callers would then have to call `riak_core_ring:num_partitions` themselves....