getClientIP was deprecated in Twisted 18.4.0, which also added
getClientAddress. The Synapse minimum version for Twisted is
currently 18.9.0, so all supported versions have the new API.
This removes the magic allowing accessing configurable
variables directly from the config object. It is now required
that a specific configuration class is used (e.g. `config.foo`
must be replaced with `config.server.foo`).
This avoids the overhead of searching through the various
configuration classes by directly referencing the class that
the attributes are in.
It also improves type hints since mypy can now resolve the
types of the configuration variables.
Patrick Cloke