* make 'event_id' a required parameter in federated state requests

As per the spec: https://matrix.org/docs/spec/server_server/r0.1.1.html#id40



Signed-off-by: Joseph Weston <joseph@weston.cloud>

* add changelog entry for bugfix

Signed-off-by: Joseph Weston <joseph@weston.cloud>

* Update server.py