Make sure that we close db connections opened during init

fix #3445 - do not use itervalues() on SortedDict()

itervalues(d) calls d.itervalues() [PY2] and d.values() [PY3]
but SortedDict only implements d.values()

don't return non-LL-member state in incremental sync state blocks

This gives clearer messages when someone gets it wrong

We should explicitly close any db connections we open, because failing to do so
can block other transactions as per
https://github.com/matrix-org/synapse/issues/3682.

Let's also try to factor out some of the boilerplate by having server classes
define their datastore class rather than duplicating the whole of `setup`.

Change admin_uri to admin_contact in config and errors

Fix checking if service notice room is already tagged

This manifested in synapse repeatedly setting the tag for the room

Fix up tagging server notice rooms.

Allow federation_domain_whitelist to be emtpy list

Outbound federation were incorrectly allowed when the config option was
set to an empty list

Fix bug where we broke sync when using limit_usage_by_mau

Pin to twisted 17.1 or later

We assumed that we always had service notices configured, but that is
not always true

Implement trial users

Fixes https://github.com/matrix-org/synapse/issues/3741.

Fix bug where we resent "limit exceeded" server notices

This was due to a bug where we mutated a cached event's contents

Reference that the federation_reader needs the HTTP replication port set

Fix MAU invalidation due to missing yield