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.
Adds missing type hints to methods in the synapse.handlers
module and requires all methods to have type hints there.
This also removes the unused construct_auth_difference method
from the FederationHandler.
Richard van der Hoff