As per https://github.com/matrix-org/matrix-doc/issues/1675

Fixes https://github.com/matrix-org/synapse/issues/2782

Remove nuke-room-from-db.sh script

The problem with this script is that it is largely untested, entirely
unmaintained, and running it is likely to make your synapse blow up in
exciting ways.

For example, it leaves a bunch of tables with dead values in it, like
event_to_state_groups.

Having it here sends a message that it is a supported part of
synapse, which is absolutely not the case.

Turns out deferred.cancel sometimes throws, so we do that last to ensure
that we always do resolve the new deferred.

Add an awful secondary timeout to fix wedged requests

This is an attempt to mitigate #3842 by adding yet-another-timeout

don't filter membership events based on history visibility
as we will already have filtered the messages in the timeline, and state events
are always visible.

and because @erikjohnston said so.

timeouts 2: electric boogaloo

Add in flight real time metrics for Measure blocks

Fix minor typo in exception

Make purge history slightly faster

Fix handling of redacted events from federation

* fix

* changelog

If we receive an event that doesn't pass their content hash check (e.g.
due to already being redacted) then we hit a bug which causes an
exception to be raised, which then promplty stops the event (and
request) from being processed.

This effects all sorts of federation APIs, including joining rooms with
a redacted state event.