Performant way to extract locality information and shard boundaries

[panghy]

Currently the only way to get the full picture of where shards are located is to get the shards (that's relatively fast, a single iteration through a system key space) and then ask for the storage servers for each of those shard one-by-one. It doesn't help that asking for IP addresses must be throttled so that you don't cause the responsible stateless processes to become pegged. There should be an easier way to accomplish this.

We need this API mainly to build coprocessors that live on the same machine as the DB and needing to figure out what data is local.

[davidscherer]

Since the locality API was originally designed, the underlying data in the system keyspace has changed to have more indirection. Possibly we should expose an API that looks more like what is actually there, or that returns boundaries, ids, and addresses for a range in a single query.