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).