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.

synapse/handlers/register.py

@@ -425,14 +425,7 @@ class RegistrationHandler(BaseHandler):
         if not address:
             return
 
-        time_now = self.clock.time()
-
-        self.ratelimiter.ratelimit(
-            address,
-            time_now_s=time_now,
-            rate_hz=self.hs.config.rc_registration.per_second,
-            burst_count=self.hs.config.rc_registration.burst_count,
-        )
+        self.ratelimiter.ratelimit(address)
 
     def register_with_store(
         self,