Description of Changes

This adds the new message types for being able to subscribe and unsubscribe to individual queries without affecting other subscriptions. Much of this was taken from #1997, but this version does not remove the existing way of subscribing to a set of queries. This renames that to legacy_ in many places, and we eventually want to get rid of it, but by making this an additive change we don't need to update all the clients at the same time.

This biggest changes are in the SubscriptionManager code. Previously we just stored two mappings:

1. Query hash to the set of clients subscribing, and
2. Table id to the set of related queries

Now we also store a mapping from client to the set of subscriptions for that client (which is useful for unsubscribing), and we keep track of both legacy subscriptions (which have a set of subscriptions per client), and the new subscriptions (which are identified by a (ClientId, RequestId) pair).

API and ABI breaking changes

This is additive, and only adds new enum types to existing messages, so this is not breaking. In the future we will want to make a breaking change to remove the legacy versions.

Expected complexity level and risk

2. This will add some complexity to the client, but this is mostly just tweaking the existing logic.

Testing

This passes existing tests (which exercise the legacy versions), and there are a few basic tests for the new version.

More unit test coverage for incremental subscription changes would be good, but I will probably add that when I start adding client support for it.

Follow-up work

We still need:

1. Client support for this.
2. More testing
3. Handling of module hot swapping. If a module is republished, and a change to indexes breaks any queries, we need to send errors to clients for those queries.