The aim here is to move the command handling out of the TCP protocol classes and to also merge the client and server command handling (so that we can reuse them for redis protocol). This PR simply moves the client paths to the new `ReplicationCommandHandler`, a future PR will move the server paths too.

Fixes #6815

Before figuring out whether we should alert a user on MAU, we call get_notice_room_for_user to get some info on the existing server notices room for this user. This function, if the room doesn't exist, creates it and invites the user in it. This means that, if we decide later that no server notice is needed, the user gets invited in a room with no message in it. This happens at every restart of the server, since the room ID returned by get_notice_room_for_user is cached.

This PR fixes that by moving the inviting bit to a dedicated function, that's only called when the server actually needs to send a notice to the user. A potential issue with this approach is that the room that's created by get_notice_room_for_user doesn't match how that same function looks for an existing room (i.e. it creates a room that doesn't have an invite or a join for the current user in it, so it could lead to a new room being created each time a user syncs), but I'm not sure this is a problem given it's cached until the server restarts, so that function won't run very often.

It also renames get_notice_room_for_user into get_or_create_notice_room_for_user to make what it does clearer.

Log warning when filesystem path is used.

Signed-off-by: Martin Milata <martin@martinmilata.cz>

Let's just call `getrusage` once on each logcontext change, rather than twice.

Only run one background update at a time

By running this stuff with `run_in_background`, it won't be correctly reported
against the relevant CPU usage stats.

Fixes #7202

This reverts commit 0122ef10.

This reverts commit 8d4cbdea.

Synapse 1.12.3 (2020-04-03)
===========================

- Remove the the pin to Pillow 7.0 which was introduced in Synapse 1.12.2, and
correctly fix the issue with building the Debian packages. ([\#7212](https://github.com/matrix-org/synapse/issues/7212))

Synapse 1.12.2 (2020-04-02)
===========================

This release fixes [an
issue](https://github.com/matrix-org/synapse/issues/7208) with building the
debian packages.

No other significant changes since 1.12.1.

This was incorrectly merged to master.

This reverts commit 319c41f5, reversing
changes made to 229eb814.

This was incorrectly merged to `master` instead of develop.

This reverts commit 90246344.

Occasionally we could get a federation device list update transaction which
looked like:

```
[
    {'edu_type': 'm.device_list_update', 'content': {'user_id': '@user:test', 'device_id': 'D2', 'prev_id': [], 'stream_id': 12, 'deleted': True}},
    {'edu_type': 'm.device_list_update', 'content': {'user_id': '@user:test', 'device_id': 'D1', 'prev_id': [12], 'stream_id': 11, 'deleted': True}},
    {'edu_type': 'm.device_list_update', 'content': {'user_id': '@user:test', 'device_id': 'D3', 'prev_id': [11], 'stream_id': 13, 'deleted': True}}
]
```

Having `stream_ids` which are lower than `prev_ids` looks odd. It might work
(I'm not actually sure), but in any case it doesn't seem like a reasonable
thing to expect other implementations to support.

* master:
  1.12.1
  Note where bugs were introduced
  1.12.1rc1
  Newsfile
  Rewrite changelog
  Add changelog
  Only import sqlite3 when type checking
  Fix another instance
  Only setdefault for signatures if device has key_json
  Fix starting workers when federation sending not split out.
  Attempt to clarify Python version requirements (#7161)
  Improve the UX of the login fallback when using SSO (#7152)
  Update the wording of the config comment
  Lint
  Changelog
  Regenerate sample config
  Whitelist the login fallback by default for SSO

Synapse 1.12.1 (2020-04-02)
===========================

No significant changes since 1.12.1rc1.

Synapse 1.12.1rc1 (2020-03-31)
==============================

Bugfixes
--------

- Fix starting workers when federation sending not split out. ([\#7133](https://github.com/matrix-org/synapse/issues/7133)). Introduced in v1.12.0.
- Avoid importing `sqlite3` when using the postgres backend. Contributed by David Vo. ([\#7155](https://github.com/matrix-org/synapse/issues/7155)). Introduced in v1.12.0rc1.
- Fix a bug which could cause outbound federation traffic to stop working if a client uploaded an incorrect e2e device signature. ([\#7177](https://github.com/matrix-org/synapse/issues/7177)). Introduced in v1.11.0.

* tag 'v1.12.1':
  1.12.1
  Note where bugs were introduced
  1.12.1rc1
  Newsfile
  Rewrite changelog
  Add changelog
  Only import sqlite3 when type checking
  Fix another instance
  Only setdefault for signatures if device has key_json
  Fix starting workers when federation sending not split out.

This broke in a recent PR (#7024) and is no longer useful due to all
replication clients implicitly subscribing to all streams, so let's
just remove it.

returning a None or an int that we don't use is confusing.

This mostly just reduces the amount of "running from sentinel context" spam
during unittest setup.

(Almost) everywhere that uses it is happy with an awaitable.

This was only used in a unit test, so let's just inline it in the test.