* Kill off redundant SynapseRequestFactory
We already get the Site via the Channel, so there's no need for a dedicated
RequestFactory: we can just use the right constructor.
* Workaround for error when fetching notary's own key
As a notary server, when we return our own keys, include all of our signing
keys in verify_keys.
This is a workaround for #6596.
(instead of everywhere that writes a response. Or rather, the subset of places
which write responses where we haven't forgotten it).
This also means that we don't have to have the mysterious version_string
attribute in anything with a request handler.
Unfortunately it does mean that we have to pass the version string wherever we
instantiate a SynapseSite, which has been c&ped 150 times, but that is code
that ought to be cleaned up anyway really.
Add federation_domain_whitelist
gives a way to restrict which domains your HS is allowed to federate with.
useful mainly for gracefully preventing a private but internet-connected HS from trying to federate to the wider public Matrix network
