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

synapse/config/ratelimiting.py

@@ -23,7 +23,7 @@ class RateLimitConfig(object):
 
 class FederationRateLimitConfig(object):
     _items_and_default = {
-        "window_size": 10000,
+        "window_size": 1000,
         "sleep_limit": 10,
         "sleep_delay": 500,
         "reject_limit": 50,
@@ -54,7 +54,7 @@ class RatelimitConfig(Config):
 
         # Load the new-style federation config, if it exists. Otherwise, fall
         # back to the old method.
-        if "federation_rc" in config:
+        if "rc_federation" in config:
             self.rc_federation = FederationRateLimitConfig(**config["rc_federation"])
         else:
             self.rc_federation = FederationRateLimitConfig(