This commit uses the race function from the async library to fix a bug where exceptions raised from the server async wouldn't be caught by the top-level code.

The bug was stemming from the fact that runAllWorkers is blocking, despite its usage of forConcurrently_. Therefore we were never actually running the wait function below, but rather we were hanging waiting on the result of the first function.

As a result the server could die but the workers could keep the main thread alive, causing the bug we just saw as part of #463.

@cgenie The usage of forConcurrently & co can be a bit subtle, but as the documentation states it means that it will run the inner action concurrently, discarding the results, but that still implies that the inner actions have to terminate!.

That wasn't the case for runAllWorkers, which is still a blocking operation. Probably worth a comment, which I will add in a follow-up commit shortly.