Fixes https://github.com/matrix-org/synapse/issues/7683

Broke in: #7649

We had a `yield` acting on a coroutine. To be fair this one is a bit difficult to notice as there's a function in the middle that just passes the coroutine along.

Enable 3PID add/bind/unbind endpoints on r0 routes

fix a typo

This reverts commit 8587b042.

* Ensure account data stream IDs are unique.

The account data stream is shared between three tables, and the maximum
allocated ID was tracked in a dedicated table. Updating the max ID
happened outside the transaction that allocated the ID, leading to a
race where if the server was restarted then the same ID could be
allocated but the max ID failed to be updated, leading it to be reused.

The ID generators have support for tracking across multiple tables, so
we may as well use that instead of a dedicated table.

* Fix bug in account data replication stream.

If the same stream ID was used in both global and room account data then
the getting updates for the replication stream would fail due to
`heapq.merge(..)` trying to compare a `str` with a `None`. (This is
because you'd have two rows like `(534, '!room')` and `(534, None)` from
the room and global account data tables).

Fix is just to order by stream ID, since we don't rely on the ordering
beyond that. The bug where stream IDs can be reused should be fixed now,
so this case shouldn't happen going forward.

Fixes #7617

Fixes https://github.com/matrix-org/synapse/issues/3177

* Clarify how to authenticate
* path params are not the same thing as query params
* Fix documentation for `/_synapse/admin/v2/users/<user_id>`

These things don't return Deferreds.

Allow new users to be registered via the admin API even if the monthly active user limit has been reached (#7263)

- Admin is able to
  - change displaynames
  - delete devices
  - list devices
  - get device informations

Fixes #7330

Closes #4382

Signed-off-by: Maxim Plotnikov <wgh@torlan.ru>

While working on https://github.com/matrix-org/synapse/issues/5665 I found myself digging into the `Ratelimiter` class and seeing that it was both:

* Rather undocumented, and
* causing a *lot* of config checks

This PR attempts to refactor and comment the `Ratelimiter` class, as well as encourage config file accesses to only be done at instantiation. 

Best to be reviewed commit-by-commit.

@uhoreg has confirmed these were both typos. They are only in comments and tests though, rather than anything critical.

Introduced in:

* https://github.com/matrix-org/synapse/pull/7157
* https://github.com/matrix-org/synapse/pull/5726

docs, default configs, comments. Nothing very significant.

Based on #7619 

async's `get_user_id_by_threepid` and its call stack.

Calls `self.get_success` on all deferred methods instead of abusing `self.pump()`. This has the benefit of working with coroutines, as well as checking that method execution completed successfully.

There are also a few small cleanups that I made in the process.

This is so the user is warned about the username not being valid as soon as possible, rather than only once they've finished UIA.

We already caught some exceptions, but not all.

Upstream have merged our changes
(https://github.com/spotify/dh-virtualenv/pull/300), so let's switch back to it
instead of using our fork.