Fixes#9490
This will break a couple of SyTest that are expecting failures to be added to the response of a federation /send, which obviously doesn't happen now that things are asynchronous.
Two drawbacks:
Currently there is no logic to handle any events left in the staging area after restart, and so they'll only be handled on the next incoming event in that room. That can be fixed separately.
We now only process one event per room at a time. This can be fixed up further down the line.
It seems most of these blacklisted tests do actually pass most of the time.
I'm of the opinion that having them blacklisted here means there is very little incentive for us to deflake any flaky tests, and meanwhile any value in those tests is completely lost.
Implements MSC2290. This PR adds two new endpoints, /unstable/account/3pid/add and /unstable/account/3pid/bind. Depending on the progress of that MSC the unstable prefix may go away.
This PR also removes the blacklist on some 3PID tests which occurs in #6042, as the corresponding Sytest PR changes them to use the new endpoints.
Finally, it also modifies the account deactivation code such that it doesn't just try to deactivate 3PIDs that were bound to the user's account, but any 3PIDs that were bound through the homeserver on that user's account.
