Workaround for error when fetching notary's own key (#6620)

* 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.