Implement message retention policies (MSC1763)

Refactor _update_auth_events_and_context_for_auth

Guess I only tested this on python 2 :/

Fixes #6419.

Fix guest registration 

Fix phone home stats

Fix caching devices for remote servers in worker.

There are lots of words in the comment as to why this is a good idea.

Fixes #6403.

Synapse 1.6.0rc2 (2019-11-25)
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Bugfixes
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- Fix a bug which could cause the background database update hander for event labels to get stuck in a loop raising exceptions. ([\#6407](https://github.com/matrix-org/synapse/issues/6407))

Add some exception handling here so that events whose json cannot be parsed are
ignored rather than getting us stuck in a loop.

Fixes #6404.

Test if a purge can make /messages return 500 responses

* Add working build command for docker image

* Add changelog

Only do `rc_login` ratelimiting on succesful login.