Fix Dart invoking medea-jason on non-main thread

[alexlapa]

Revealed in #193

Synopsis

From #193 it seems that at least two different threads call executor task_wake. medea-jason is designed to work in a single threaded environment so that might end bad.

Solution

It was tracked that second thread appears at callback_finalizer and that makes sense since Dart does not guarantee that this callback will only be called on a main thread:

If the object becomes unreachable the callback is invoked with the
the peer as argument. The callback can be executed on any thread, will have
an isolate group, but will not have a current isolate. The callback can only
call Dart_DeletePersistentHandle or Dart_DeleteWeakPersistentHandle.

Checklist

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[alexlapa]

FCM

Fix `Future`s executor being called from non-main thread on Dart platforms (#197)

Additionally:
- use `std`'s `Waker` instead of custom `RawWaker` implementation
- fix `CLIENT_API_ADDR` config value extracting in the Flutter demo