Alternative/enhanced API for application services to do presence

Application services, such as https://github.com/Half-Shot/matrix-appservice-discord, may want bridge presence from the remote service. The current system requires the bridge to spam updates constantly just to keep the user's presence updated. This doesn't scale nicely when your bridge hits thousands of remote users being bridged.

Instead, a transactional approach may be better suited: Provided a bridge is responding, assume all presence updates are valid until otherwise identified. There may need to be some other checks in place to assist against the bridge forgetting presence status, although that issue could easily be shifted to the author of the bridge.

Start by reviewing the current Matrix presence behavior for application services and the proposed transactional approach described in the issue. Done would require a defined alternative API, clear rules for validity and bridge responsiveness, and agreement on how forgotten presence status is handled.