Use a producer to stream back responses

Refactor request logging code

add new error type ResourceLimit

Merge branch 'neilj/limit_exceeded_error' of github.com:matrix-org/synapse into neilj/limit_exceeded_error

Neilj/resource limit block event creation

Update the admin register documentation to return a real user ID

Presumably this is the intention anyways. I've also updated the domain part to be something more along the lines of what people might expect.

Fix inbound federation on reader worker

Inbound federation requires calculating push, which in turn relies on
having access to account data.

Changelog should link to PRs

Fix Mau off by one errors

These were removed for the 0.33.2 release, but were incorrectly re-added in

The problem with dumping all of the json response into the Request object at
once is that doing so starts the timeout for the next request to be received:
so if it takes longer than 60s to stream back the response to the client, the
client never gets it.

The correct solution is to use a Producer; then the timeout is only started
once all of the content is sent over the TCP connection.

Move more federation APIs to workers

support admin_email config and pass through into blocking errors, 

This involves renaming _persist_events to be a public function.

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.