This commit moves a bunch of the logic for deciding when to log the receipt and
completion of HTTP requests into SynapseRequest, rather than in the request
handling wrappers.
Advantages of this are:
* we get logs for *all* requests (including OPTIONS and HEADs), rather than
just those that end up hitting handlers we've remembered to decorate
correctly.
* when a request handler wires up a Producer (as the media stuff does
currently, and as other things will do soon), we log at the point that all
of the traffic has been sent to the client.
eaaa2248ff