It turns out that the only thing we use the __dict__ of LoggingContext for is
`request`, and given we create lots of LoggingContexts and then copy them every
time we do a db transaction or log line, using the __dict__ seems a bit
redundant. Let's try to optimise things by making the request attribute
explicit.

Fix a bug where we could end up firing off multiple requests for server_keys
for the same server at the same time.

I'm still unclear on what the intended behaviour for
`verify_json_objects_for_server` is, but at least I now understand the
behaviour of most of the things it calls...

Another signing test vector using an 'm.room.message' with content, so that the implementation will have to redact it