Set Server header in SynapseRequest

(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.

synapse/app/event_creator.py

@@ -114,6 +114,7 @@ class EventCreatorServer(HomeServer):
                 site_tag,
                 listener_config,
                 root_resource,
+                self.version_string,
             )
         )