Make FederationRateLimiter queue requests properly
Make FederationRateLimiter queue requests properly

popitem removes the *most recent* item by default [1]. We want the oldest.

Fixes #3524

[1]: https://docs.python.org/2/library/collections.html#collections.OrderedDict.popitem

matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse