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.

tests/rest/client/v1/test_typing.py

@@ -16,7 +16,7 @@
 
 """Tests REST events for /rooms paths."""
 
-from mock import Mock, NonCallableMock
+from mock import Mock
 
 from twisted.internet import defer
 
@@ -39,17 +39,11 @@ class RoomTypingTestCase(unittest.HomeserverTestCase):
     def make_homeserver(self, reactor, clock):
 
         hs = self.setup_test_homeserver(
-            "red",
-            http_client=None,
-            federation_client=Mock(),
-            ratelimiter=NonCallableMock(spec_set=["can_do_action"]),
+            "red", http_client=None, federation_client=Mock(),
         )
 
         self.event_source = hs.get_event_sources().sources["typing"]
 
-        self.ratelimiter = hs.get_ratelimiter()
-        self.ratelimiter.can_do_action.return_value = (True, 0)
-
         hs.get_handlers().federation_handler = Mock()
 
         def get_user_by_access_token(token=None, allow_guest=False):