Ensure an orphaned SubstrateHarness actor can be rescued

There isn't a clear story yet how to handle orphaned Substrate harness actors. Consider the following case:

- AX server crashes before a harness actor finishes
- After harness finishes working, it won't be auto suspended by the AX server
- Furthermore, incoming requests will be rejected because the orphaned actor is still in flight

By switching to the new async communication between the server and harness, we should be able to recover and suspend after resumption. This behavior requires more thought and discussion before we start the implementation.