Add broadcast API aka watch support

[SUPERCILEX]

Add a way for clients to be notified when anything changes. Ideally the API tells you what changed but probably also needs an escape hatch to say "eh, dunno reload everything" for garbage collection for example. Previous notes: #3

[SUPERCILEX]

For the UI to be efficient, I think we need to subscribe to server broadcasts while visible, drop the connection while in the background, and statx every bucket and ring with a full reload when back in the foreground. Since the server doesn't store a log of operations, there's no way to efficiently generate a diff between two random points in time. Also when reconnecting to the server we need to subscribe to changes before doing a full refresh to avoid the case where the server GCs as we refresh.

[SUPERCILEX]

Something I just realized: for the watcher clients, they don't need to differentiate between requests and pushes. If you request an add for example, it'll show up in your event stream just like any other client requesting an add, so there's no speed to special case the receive buffer for your requests vs other people's changes.