Performance improvements and refactor of Ratelimiter (#7595)

While working on https://github.com/matrix-org/synapse/issues/5665 I found myself digging into the `Ratelimiter` class and seeing that it was both:

* Rather undocumented, and
* causing a *lot* of config checks

This PR attempts to refactor and comment the `Ratelimiter` class, as well as encourage config file accesses to only be done at instantiation. 

Best to be reviewed commit-by-commit.

1 files changed, 2 insertions, 6 deletions

diff --git a/tests/rest/client/v1/test_events.py b/tests/rest/client/v1/test_events.py

index b54b06482b..f75520877f 100644
--- a/tests/rest/client/v1/test_events.py
+++ b/tests/rest/client/v1/test_events.py
@@ -15,7 +15,7 @@
 
 """ Tests REST events for /events paths."""
 
-from mock import Mock, NonCallableMock
+from mock import Mock
 
 import synapse.rest.admin
 from synapse.rest.client.v1 import events, login, room
@@ -40,11 +40,7 @@ class EventStreamPermissionsTestCase(unittest.HomeserverTestCase):
         config["enable_registration"] = True
         config["auto_join_rooms"] = []
 
-        hs = self.setup_test_homeserver(
-            config=config, ratelimiter=NonCallableMock(spec_set=["can_do_action"])
-        )
-        self.ratelimiter = hs.get_ratelimiter()
-        self.ratelimiter.can_do_action.return_value = (True, 0)
+        hs = self.setup_test_homeserver(config=config)
 
         hs.get_handlers().federation_handler = Mock()