Also:
- change the REST endpoint to have a "S" on the end (so it's now /read_markers)
- change the content of the m.read_up_to event to have the key "event_id" instead of "marker".
This tracks data about the entity which made the request. This is
instead of passing around a tuple, which requires call-site
modifications every time a new piece of optional context is passed
around.
I tried to introduce a User object. I gave up.
