fixes #3260

Hopefully there are enough comments and docs in this that it makes sense on its
own.

We aren't ready to release this yet, so I'm reverting it for now.

This reverts commit d1679a4e, reversing
changes made to e089100c.

Fixes https://github.com/matrix-org/synapse/issues/3018



Signed-off-by: Travis Ralston <travpc@gmail.com>

Signed-off-by: Travis Ralston <travpc@gmail.com>

clarify that `purge_history` will not purge state

All the others are code blocks, so this one should be to (currently it is a blockquote).

Signed-off-by: Travis Ralston <travpc@gmail.com>

Make the purge request return quickly, and allow scripts to poll for updates.

It annoys me that we create temporary function objects when there's really no
need for it. Let's factor the gubbins out of preserve_fn and start using it.

Signed-off-by: Travis Ralston <travpc@gmail.com>

* It's supposed to be purge_local_events, not ..._history
* Fix the doc to have valid json

Add a flag which makes the purger delete local events

Signed-off-by: Travis Ralston <travpc@gmail.com>

... to stop us doing the cache cleanup jobs on the master.

Fixes https://github.com/matrix-org/synapse/issues/2554

Both of these functions ae known to leak logcontexts. Replace the remaining
calls to them and kill them off.

We're going to need to use this from places that aren't password auth, so let's
move it to a proper class.

Provide a hook by which auth providers can be notified of logouts.

... so that they have a way to record access tokens.

Provide a hook where password auth providers can say they know about other
login types, and get passed the relevant parameters

Provide an interface by which password auth providers can register db schema
files to be run at startup

Document the existing interface, before I start adding new stuff.