Fix bugs when joining a remote room that has dodgy event graphs. This should also fix the number of times a HS will trigger a GET /event/

Re-sign events when we return them via federation as a temporary hack to work around the problem where we reconstruct events differently than when they were signed

Add a workaround for bug where some initial join events don't reference creation events in their auth_events

Correctly handle the case where we get an event for an unknown room, which turns out we are actually in

We now do more implement more of the auth on the events so that we
don't reject valid events.

Drop log level for incorrect logging contexts to WARN if the context is wrong and DEBUG if the context is missing

This should fix the bug where the edges of the graph get returned in a
different order than they were inserted in, and so no get_event no
longer returned the exact same JSON as was inserted. This meant that
signature checks failed.

SYN-58: Allow passing explicit limit=0 to initialSync to request no messages at all; missing still implies default 10