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/handlers/test_profile.py

@@ -14,7 +14,7 @@
 # limitations under the License.
 
 
-from mock import Mock, NonCallableMock
+from mock import Mock
 
 from twisted.internet import defer
 
@@ -55,12 +55,8 @@ class ProfileTestCase(unittest.TestCase):
             federation_client=self.mock_federation,
             federation_server=Mock(),
             federation_registry=self.mock_registry,
-            ratelimiter=NonCallableMock(spec_set=["can_do_action"]),
         )
 
-        self.ratelimiter = hs.get_ratelimiter()
-        self.ratelimiter.can_do_action.return_value = (True, 0)
-
         self.store = hs.get_datastore()
 
         self.frank = UserID.from_string("@1234ABCD:test")