Wait for isolate to be destroyed before creating a new one

[annagrin]

We seem to have a race condition where isolate starts without an isolate destruction in the middle.

See original issue for examples: flutter/flutter#117676

In this change we try to mitigate the issue by waiting for isolate to be destroyed before creating a new one.

[elliette]

Discussed offline - we are going to try to fix the simultaneous isolates with:

1. a thread pool to prevent concurrent isolate creation events
2. calling destroyIsolate from createIsolate to guarantee the previous isolate has first been destroyed

[annagrin]

Closing in favor of #1870