Remove trailing slashes from outbound federation requests and retry on 400 (#4840)

As per #3622, we remove trailing slashes from outbound federation requests. However, to ensure that we remain backwards compatible with previous versions of Synapse, if we receive a HTTP 400 with `M_UNRECOGNIZED`, then we are likely talking to an older version of Synapse in which case we retry with a trailing slash appended to the request path.