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.
