Created the message envelope API.  Created concrete implementations.

AFDP-94
Created APIs for the request/reply queue and incoming message handler.

AFDP-94
Created a concrete implementation of the incoming message handler.

AFDP-94
Created stubbed out implementation of the request/reply queue.

AFDP-94
Created test implementation of a task result handler.

AFDP-94
Removed the notion of callbacks completely.  Updated tests.

AFDP-94