AppServices: Registration option for disallowing AS user events being sent back to the AS

Currently when an application service sends a room message through one of their fake users, say @_appservice_bob:domain.com, that username matches the application service's users regex. Due to this, the homeserver considers the application service interested in the event, and thus sends it back to the application service.

Bridges, for instance, don't care about these messages and simply drop them if they see it comes from a user who's userID matches their users regex.

An option, such as prevent_returning_sent_events, which would instruct the homeserver not to send events that have originally come from an appservice back to that same appservice, would prevent some bandwidth from being wasted here.

@Half-Shot

The issue proposes a Matrix application-service event-routing option named prevent_returning_sent_events, but names no file, specification section, or test. Start by locating the application-service event delivery rules in the specification; done means the behavior and option are clearly defined for events sent by an appservice user.