Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

Fixes #7901.

Signed-off-by: Niklas Tittjung <nik_t.01@web.de>

Fixes https://github.com/matrix-org/synapse/issues/2431

Adds config option `encryption_enabled_by_default_for_room_type`, which determines whether encryption should be enabled with the default encryption algorithm in private or public rooms upon creation. Whether the room is private or public is decided based upon the room creation preset that is used.

Part of this PR is also pulling out all of the individual instances of `m.megolm.v1.aes-sha2` into a constant variable to eliminate typos ala https://github.com/matrix-org/synapse/pull/7637

Based on #7637

@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

The idea here is that if an instance persists an event via the replication HTTP API it can return before we receive that event over replication, which can lead to races where code assumes that persisting an event immediately updates various caches (e.g. current state of the room).

Most of Synapse doesn't hit such races, so we don't do the waiting automagically, instead we do so where necessary to avoid unnecessary delays. We may decide to change our minds here if it turns out there are a lot of subtle races going on.

People probably want to look at this commit by commit.

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.

make sure we clear out all but one update for the user

this is never set to anything other than "test", and is a source of unnecessary
boilerplate.

Ensure good comprehension hygiene using flake8-comprehensions.

... and use it in places where it's trivial to do so.

This will make it easier to pass room versions into the FrozenEvent
constructors.

Ensure that the the default settings for the room directory are that the it is hidden from public view by default. 

Rate-limit outgoing read-receipts as per #4730.

... as described at
https://docs.google.com/document/d/1EttUVzjc2DWe2ciw4XPtNpUpIl9lWXGEsy2ewDS7rtw.

setup_test_homeserver function in utils.

Supply auth_chain along with current state in '/state/', fetch auth events from a remote server if we are missing some of them