A batch of changes intended to make it easier to trace to-device messages through the system.
The intention here is that a client can set a property org.matrix.msgid in any to-device message it sends. That ID is then included in any tracing or logging related to the message. (Suggestions as to where this field should be documented welcome. I'm not enthusiastic about speccing it - it's very much an optional extra to help with debugging.)
I've also generally improved the data we send to opentracing for these messages.
