* Remove references to handler._auth_handler

(and replace them with hs.get_auth_handler)

* Factor out a utility function for building Requests

* Remove mocks of `OidcHandler._map_userinfo_to_user`

This method is going away, so mocking it out is no longer a valid approach.

Instead, we mock out lower-level methods (eg _remote_id_from_userinfo), or
simply allow the regular implementation to proceed and update the expectations
accordingly.

* Remove references to `OidcHandler._map_userinfo_to_user` from tests

This method is going away, so we can no longer use it as a test point. Instead
we build mock "callback" requests which we pass into `handle_oidc_callback`,
and verify correct behaviour by mocking out `AuthHandler.complete_sso_login`.

Spam checker modules can now provide async methods. This is implemented
in a backwards-compatible manner.

* Consistently use room_id from federation request body

Some federation APIs have a redundant `room_id` path param (see
https://github.com/matrix-org/matrix-doc/issues/2330). We should make sure we
consistently use either the path param or the body param, and the body param is
easier.

* Kill off some references to "context"

Once upon a time, "rooms" were known as "contexts". I think this kills of the
last references to "contexts".

This was broken in #8801.

This is now only used in `test_typing`, so move it there.

A few test cases were relying on being able to mount non-client servlets on the
test resource. it's better to give them their own Resources.

Replaces the `federation_ip_range_blacklist` configuration setting with an
`ip_range_blacklist` setting with wider scope. It now applies to:

* Federation
* Identity servers
* Push notifications
* Checking key validitity for third-party invite events

The old `federation_ip_range_blacklist` setting is still honored if present, but
with reduced scope (it only applies to federation and identity servers).

This was broken in #8801 when abstracting code shared with OIDC.

After this change both SAML and OIDC have a concept of
grandfathering users, but with different implementations.

The idea here is to abstract out all the conditional code which tests which
methods a given password provider has, to provide a consistent interface.

The spec requires synapse to support `identifier` dicts for `m.login.password`
user-interactive auth, which it did not (instead, it required an undocumented
`user` parameter.)

To fix this properly, we need to pull the code that interprets `identifier`
into `AuthHandler.validate_login` so that it can be called from the UIA code.

Fixes #5665.

Fix a minor bug where we would offer "m.login.password" login if a custom auth provider supported it, even if password login was disabled.

These things seemed to be completely untested, so I added a load of tests for
them.

Changes `@cache_in_self` to use underscore-prefixed attributes.

Abstracts the SAML and OpenID Connect code which attempts to regenerate
the localpart of a matrix ID if it is already in use.

Checks that the localpart returned by mapping providers for SAML and
OIDC are valid before registering new users.

Extends the OIDC tests for existing users and invalid data.

* Consistently use room_id from federation request body

Some federation APIs have a redundant `room_id` path param (see
https://github.com/matrix-org/matrix-doc/issues/2330). We should make sure we
consistently use either the path param or the body param, and the body param is
easier.

* Kill off some references to "context"

Once upon a time, "rooms" were known as "contexts". I think this kills of the
last references to "contexts".

De-duplicates code between the SAML and OIDC implementations.

Fixes #4042

We do it this way round so that only the "owner" can delete the access token (i.e. `/logout/all` by the "owner" also deletes that token, but `/logout/all` by the "target user" doesn't).

A future PR will add an API for creating such a token.

When the target user and authenticated entity are different the `Processed request` log line will be logged with a: `{@admin:server as @bob:server} ...`. I'm not convinced by that format (especially since it adds spaces in there, making it harder to use `cut -d ' '` to chop off the start of log lines). Suggestions welcome.

#8567 started a span for every background process. This is good as it means all Synapse code that gets run should be in a span (unless in the sentinel logging context), but it means we generate about 15x the number of spans as we did previously.

This PR attempts to reduce that number by a) not starting one for send commands to Redis, and b) deferring starting background processes until after we're sure they're necessary.

I don't really know how much this will help.

* Fix outbound federaion with multiple event persisters.

We incorrectly notified federation senders that the minimum persisted
stream position had advanced when we got an `RDATA` from an event
persister.

Notifying of federation senders already correctly happens in the
notifier, so we just delete the offending line.

* Change some interfaces to use RoomStreamToken.

By enforcing use of `RoomStreamTokens` we make it less likely that
people pass in random ints that they got from somewhere random.

Update `EventCreationHandler.create_event` to accept an auth_events param, and
use it in `_locally_reject_invite` instead of reinventing the wheel.

this is always the same as requester.access_token_id.

Should fix #3365.

All handlers now available via get_*_handler() methods on the HomeServer.

This allows a user to store an offline device on the server and
then restore it at a subsequent login.

This is now redundant, and we can just call `handle_new_client_event` directly.

This allows for connecting to certain IdPs, e.g. GitLab.