Refactor storage layer to support multiple databases

Cross-signing [3/4] -- uploading signatures edition

The synapse demo was a bit flakey in terms of supporting federation. It turns out that if your computer resolved `localhost` to `::1` instead of `127.0.0.1`, the built-in federation blacklist specified in `start.sh` would still block it, since it contained an entry for `::/127`. Removing this no longer prevents Synapse from contacting `::1`, federation works again on these boxes.

Added database owner authentication with `sudo` when `su` does not work

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

* Delete format_tap.py

This python implementation of a tap formatting library for buildkite has been
replaced with a perl implementation as part of the matrix-org/sytest repo,
which is specific to sytest's language, not that of any one homeserver's.

... again?  How did you make it disappear, git?

This method was somewhat redundant, and confusing.

Remove a bunch of dead event_auth code.

Turns out that loggers that are instantiated before the config is loaded get
turned off.

Also bring the logging config that is generated by --generate-config into line.

Fixes #6194.

Synapse 1.4.1 (2019-10-18)
==========================

No changes since 1.4.1rc1.

Synapse 1.4.1rc1 (2019-10-17)
=============================

Bugfixes
--------

- Fix bug where redacted events were sometimes incorrectly censored in the database, breaking APIs that attempted to fetch such events. ([\#6185](https://github.com/matrix-org/synapse/issues/6185), [5b0e9948](https://github.com/matrix-org/synapse/commit/5b0e9948eaae801643e594b5abc8ee4b10bd194e))

Move rest/admin to use async/await.

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

* 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.

The only possible rejection reason is AUTH_ERROR, so all of this is unreachable.

This is part of 1.4.1