Consider possible race condition when registering keys

A race condition is possible when registering meta keys across requests to the same collection. Consider an alternative approach to storage that uses sharded meta keys to store keys.

From @dlh01 in #11:

> The use of a single array of keys is still, I think, subject to the race condition that you and Boynes discussed in Slack. However, using the metadata API allows for a possible workaround: for each key, store a separate meta row. That way, concurrent requests would write to their own rows, not the same row? I haven't played that all the way out in my head, so maybe it's a dead end.