Allow disconnecting in fetchCredentials callback

[simolus3]

To prevent duplicate concurrent sync connections, we guard connect() and disconnect() calls in mutexes. And since the mutex implementation is not reentrant, there's a check ensuring another call to .lock() throws if it comes from an async zone that already holds the mutex.

By design, async zones are quite sticky: Every callback we register in that zone will also run in the same zone. So when we call connectInternal on the native database, all message handlers from the isolate are handled in the zone originally establishing the connection.

This means that these callbacks can't call disconnect(), which is unfortunate because the lock is not actually held at that point. This fixes that issue by passing the zone originally calling connect() which is then used to run callbacks.
This allows calling disconnect() in response to tokens expiring and fetchCredentials being called (as long as that call is not awaited - disconnect() will await the abortion which is then blocked on the disconnect() call itself).

Closes #278.