This was ill-advised. We can't modify verify_keys here, because the response
object has already been signed by the requested key.
Furthermore, it's somewhat unnecessary because existing versions of Synapse
(which get upset that the notary key isn't present in verify_keys) will fall
back to a direct fetch via `/key/v2/server`.
Also: more tests for fetching keys via perspectives: it would be nice if we actually tested when our fetcher can't talk to our notary impl.
* 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
