The old statement returned `None` for such a `password_config` (like the one
created on first run), thus retrieval of the `pepper` key failed with
`AttributeError`.

Fixes #5315

Signed-off-by: Ivan Vilata i Balaguer <ivan@selidor.net>

* Raise an exception if there are pending background updates

So we return with a non-0 code

* Changelog

* Port synapse_port_db to async/await

* Port update_database to async/await

* Add version string to mocked homeservers

* Remove unused imports

* Convert overseen bits to async/await

* Fixup logging contexts

* Fix imports

* Add a way to print an error without raising an exception

* Incorporate review

Currently we rely on `current_state_events` to figure out what rooms a
user was in and their last membership event in there. However, if the
server leaves the room then the table may be cleaned up and that
information is lost. So lets add a table that separately holds that
information.

We might not need the cursor at all.

* Add a background update to clear tombstoned rooms from the directory

* use the ABC metaclass

I want to do some key rotation, and it is silly that we don't have a way to do
this.

This encapsulates config for a given database and is the way to get new
connections.

Replace every instance of `logger.warn` with `logger.warning` as the former is deprecated.

Make `synapse_port_db` correctly create indexes in the PostgreSQL database, by having it run the background updates on the database before migrating the data.

To ensure we're migrating the right data, also block the port if the SQLite3 database still has pending or ongoing background updates.

Fixes #4877 

Small fix to synapse_port_db to be able to convert from database schema v56.

Fixes #5306

Tables dropped: 
 * application_services, 
 * application_services_regex, 
 * transaction_id_to_pdu, 
 * stats_reporting
 * current_state_resets
 * event_content_hashes
 * event_destinations
 * event_edge_hashes
 * event_signatures
 * feedback
 * room_hosts
 * state_forward_extremities