Connect message handlers to message work tracking

Message handlers are currently naive in that a consumer performs `xreadgroup`, grabs a message, and calls all the functions on it. Functions like `update_handler_completion` in `event_stream.bus` need to be called around the message processing in order to ensure operations that failed may be attempted again.

Start in event_stream.bus with update_handler_completion, then trace the consumer path that performs xreadgroup and invokes message functions. Connect message processing to the work-tracking calls so failed operations can be attempted again; done means failed work is retried rather than silently remaining incomplete.

Written by the indexing model from the issue text.