Add `streams` to `PerAccountStore`, with a map keyed by stream ID

[chrisbobbe]

We should initialize this from the streams key in the initial snapshot (InitialSnapshot), and keep it updated in response to stream events.

[chrisbobbe]

Assigning Greg because he mentioned having a local branch that accomplishes this.

[gnprice]

My branch (which I'll send as a PR shortly) adds InitialSnapshot.streams, and adds a map on PerAccountStore, but doesn't add the event-handling to keep the map updated.

I think that's fine for the moment; it's how the subscriptions map already works. We can let #135 track the need to add event-handling to update this data.