Make mitm/callback options more composable

[kegsay]

It's slightly better but not perfect yet because:

• you can't enable both blocking and callbacks, then drop the blocking but not the callbacks. TestToDeviceMessagesAreProcessedInOrder needs this to block /sync for a while then unblock it, and throughout it wants to listen for /sync responses.
• You can't listen for >1 path with a MITMConfiguration. We don't have any tests which want this currently though.
• You can't do funky logic to determine if you should block a request (e.g only block if the request has these fields set). Again, no tests need this currently.

Partly addresses #68

[andybalaam]

Yep, much nicer.