... and use it in places where it's trivial to do so.

This will make it easier to pass room versions into the FrozenEvent
constructors.

and associated functions:
 * on_receive_pdu
 * handle_queued_pdus
 * get_missing_events_for_pdu

The intention here is to make it clearer which fields we can expect to be
populated when: notably, that the _event_type etc aren't used for the
synchronous impl of EventContext.

This is in preparation to refactor FrozenEvent to support different
event formats for different room versions

This test didn't do what it claimed to do, and what it claimed to do was the
same as test_cant_hide_direct_ancestors anyway.

This stuff is tested by sytest anyway.

when processing incoming transactions, it can be hard to see what's going on,
because we process a bunch of stuff in parallel, and because we may end up
recursively working our way through a chain of three or four events.

This commit creates a way to use logcontexts to add the relevant event ids to
the log lines.