Port synapse_port_db to async/await (#6718)

* 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