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