Convert synapse.federation.transport.server to async (#5689)

* Convert BaseFederationServlet._wrap to async

Empirically, this fixes some lost stacktraces. It should be safe because the
wrapped function is called from JsonResource._async_render, which is already
async.

* Convert the rest of synapse.federation.transport.server to async

We may as well do the whole file while we're here.

* changelog

* flake8

changelog.d/5689.misc

@@ -0,0 +1 @@
+Convert `synapse.federation.transport.server` to `async`. Might improve some stack traces.