Convert Mirror to be non-suspendable

[gavin-norman-sociomantic]

Rationale:

• I don't think there's a real use case for suspending a Mirror.
• Suspending a Mirror doesn't really work properly, as it is highly likely to lead to the node's mirror queues overflowing and updates being lost. Apps that use Mirror should be able to handle the data it provides without needing to suspend.
• (A nice side-effect:) This will greatly simplify the request handling code in the client.

The request needs to be cleanly stoppable, still.