Configure Renovate

See merge request !1

Make 'app_key' into a proper secondary key, completely independent
from the main shard/type/resource_id key, providing a hash map for
each type/resource_id combination.

The Get() call semantics are extended to support scanning the app_key
space when passing an empty 'app_key' attribute.

Keys in the API are now explicit, not represented as strings (the
string encoding no longer has any usage except debugging).

We can now run sharded queries on the server itself, simplifying the
client implementation. The server will forward the request to all
desired shards, it will store the data in a temporary SQL table, and
it will then run a *reduce* query on it to return the results to the
caller.