As per MSC1763, 'Retention is only considered for non-state events.', so don't filter out state events based on the room's retention policy.

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.