Reduce federation replication traffic

Add a counter metric for successfully-sent transactions

Dedupe KeyedEdu and Devices federation repl traffic

Remove HTTP replication APIs

Fix getting latest device IP for user with no devices

This is mainly done by moving the calculation of where to send presence
updates from the presence handler to the transaction queue, so we only
need to send the presence event (and not the destinations) across the
replication connection. Before we were duplicating by sending the full
state across once per destination.

Support authenticated SMTP

Fix up federation SendQueue and document types

When we do an invite rejection, save the signed leave event to the db

During a rejection of an invite received over federation, we ask a remote
server to make us a `leave` event, then sign it, then send that with
`send_leave`.

We were saving the *unsigned* version of the event (which has a different event
id to the signed version) to our db (and sending it to the clients), whereas
other servers in the room will have seen the *signed* version. We're not aware
of any actual problems that caused, except that it makes the database confusing
to look at and generally leaves the room in a weird state.