Completion handlers for delivery state changes

[mender3]

Description

Include async result handlers on delivery state change methods, accepted, released, rejected, & modified.

Use case

Message consumer completes processing a message and calls the accepted method. The broker fails to receive or handle the state change for some reason. That message could potentially be redelivered and reprocessed. An async result handler would allow the message consumer to take appropriate action in the event of a failure, or move on with confidence that the message's lifecycle is complete.