Fix distributing table updates from shared worker

[simolus3]

In #105, the throttledUpdatedTables stream was shared between different connections connecting to a database worker. That's a problem, because the've been implemented as a single-subscription stream, so only the first subscription would have been successful.

This:

1. Replaces throttledUpdatedTables with a non-throttled variant that supports multiple subscriptions. The stream can still be throttled by pausing the subscription, which will emit items into a pending set internally.
2. On native platforms, removes the throttle entirely. Sending isolate messages isn't that expensive, we only do it once per transaction, and there's still the regular throttling logic for watched queries downstream.
3. On the web, we pause the subscription while the client is busy handling the notification, giving us backpressure support without a fixed delay after each message.

I've tested package:powersync_core web tests with these changes and also ran a demo app with this to ensure it works.

[stevensJourney]

The logic looks solid to me.