Simplify control flow in sync client

[simolus3]

This closes #409. Previously, there was a potential race condition when adding new sync streams because we:

1. First added a AbortCurrentIteration local sync event.
2. Which is then handled asynchronously to break out of the client loop once handled, but it's possible for additional server-side events to come in before we get to that.

Instead of handling close events asynchronously, this refactors the control flow of the sync client, allowing us to just break out of a loop when the core extension tells us to instead of having to go through these hoops. We do this by making all instructions that won't interrupt a sync iteration extend a common class, so we can safely express which instructions we can handle in a separate methods and which ones need to be part of the loop.

This also updates the core extension and removes a workaround for a package:http bug related to cancelling streams on the web.

[stevensJourney]

The changes here look good to me :)