Ability to cancel to-device msgs

Both matrix-org/matrix-spec-proposals#3401 and E2EE performance would benefit from the ability to cancel to-device messages sent to a user once they have been acknowledged somehow from a single device.

For matrix-org/matrix-spec-proposals#3401, this would allow irrelevant call invites to be dropped on other devices once a call has been answered on one device. For E2EE this could radically speed up sync time after a gap on a given client, as we could ditch megolm keys sent to a given device if we know we have stored them in online backup (and then the other devices could fish them out of backup instead).

Start by reading the issue and matrix-org/matrix-spec-proposals#3401, then review how acknowledged to-device messages are described in the Matrix specification. Define the protocol behavior needed to cancel messages safely across a user's devices, including the call-invite and E2EE key scenarios; done means the required specification changes and interoperability rules are agreed.

Written by the indexing model from the issue text.