And fix the tests to actually test that things got deleted.

Fixes a bug where rejected events were persisted with the wrong state group.

Also fixes an occasional internal-server-error when receiving events over
federation which are rejected and (possibly because they are
backwards-extremities) have no prev_group.

Fixes #6289.

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

... to stop people causing DoSes with malicious web pages

* remove psutil and replace with resource

The intention here is to make it clearer which fields we can expect to be
populated when: notably, that the _event_type etc aren't used for the
synchronous impl of EventContext.

The `http_proxy` and `HTTPS_PROXY` env vars can be set to a `host[:port]` value which should point to a proxy.

The address of the proxy should be excluded from IP blacklists such as the `url_preview_ip_range_blacklist`.

The proxy will then be used for
 * push
 * url previews
 * phone-home stats
 * recaptcha validation
 * CAS auth validation

It will *not* be used for:
 * Application Services
 * Identity servers
 * Outbound federation
 * In worker configurations, connections from workers to masters

Fixes #4198.

This makes it easier to use in an async/await world.

Also fixes a bug where cache descriptors would occaisonally return a raw
value rather than a deferred.

The expected use case is to suppress MAU limiting on small instances

This is in preparation for having multiple data stores that offer
different functionality, e.g. splitting out state or event storage.

* Fix presence timeouts when synchrotron restarts.

Handling timeouts would fail if there was an external process that had
timed out, e.g. a synchrotron restarting. This was due to a couple of
variable name typoes.

Fixes #3715.