CORE-5509: Clean up DE service code.
CORE-5509: Reinstated DE callback wrapper.
This was done by injecting a new class which wraps the given callbacks in an AsyncCallbackWrapper class. The DEServiceAsync RPC is no longer used directly.
This is, essentially, re-creating the facade wrapping pattern which was recently removed. However, the other alternative to this is to manually wrap the callbacks given each DEServiceAsync one-by-one.

This is a heavy handed approach to handling redirects through the AsyncCallbackWrapper. Ideally, the functionality provided by AsyncCallbackWrapper should be handled in a more graceful manner.