Fixes to the federation rate limiter (#5621)

- Put the default window_size back to 1000ms (broken by #5181)
- Make the `rc_federation` config actually do something
- fix an off-by-one error in the 'concurrent' limit
- Avoid creating an unused `_PerHostRatelimiter` object for every single
  incoming request

tests/utils.py

@@ -152,12 +152,6 @@ def default_config(name, parse=False):
         "mau_stats_only": False,
         "mau_limits_reserved_threepids": [],
         "admin_contact": None,
-        "rc_federation": {
-            "reject_limit": 10,
-            "sleep_limit": 10,
-            "sleep_delay": 10,
-            "concurrent": 10,
-        },
         "rc_message": {"per_second": 10000, "burst_count": 10000},
         "rc_registration": {"per_second": 10000, "burst_count": 10000},
         "rc_login": {