Implement updated auth rules from MSC2260

We just mark the fact that the cache may be stale in the database for
now.

These are easier to work with than the strings and we normally have one around.

This fixes `FederationHander._persist_auth_tree` which was passing a
RoomVersion object into event_auth.check instead of a string.

As using non-C locale can cause issues on upgrading OS.

Calling the invalidation function during initialisation of the data
stores introduces a circular dependency, causing Synapse to fail to
start.

This is so that we don't have to rely on pulling it out from `current_state_events` table.

Currently if a worker invalidates a cache it will be streamed to master, which then didn't forward those to other workers.

Clarifications to the workers documentation

Co-Authored-By: Brendan Abolivier <babolivier@matrix.org>

Signed-off-by: Jason Robinson <jasonr@matrix.org>

* Add note that user_dir requires disabling user dir
  updates from the main synapse process.
* Add note that federation_reader should have
  the federation listener resource.

Signed-off-by: Jason Robinson <jasonr@matrix.org>

There are quite a few places that we assume that a redaction event has a
corresponding `redacts` key, which is not always the case. So lets
cheekily make it so that event.redacts just returns None instead.

* a bit of debugging for media storage providers

* changelog

* don't insist on a password (this is valid if you have an SSO login)
* fix reference to undefined `requester`

Fix typo in _select_thumbnail