Add federation_domain_whitelist option (#2820)

Add federation_domain_whitelist

gives a way to restrict which domains your HS is allowed to federate with.
useful mainly for gracefully preventing a private but internet-connected HS from trying to federate to the wider public Matrix network

synapse/util/retryutils.py

@@ -26,6 +26,18 @@ logger = logging.getLogger(__name__)
 
 class NotRetryingDestination(Exception):
     def __init__(self, retry_last_ts, retry_interval, destination):
+        """Raised by the limiter (and federation client) to indicate that we are
+        are deliberately not attempting to contact a given server.
+
+        Args:
+            retry_last_ts (int): the unix ts in milliseconds of our last attempt
+                to contact the server.  0 indicates that the last attempt was
+                successful or that we've never actually attempted to connect.
+            retry_interval (int): the time in milliseconds to wait until the next
+                attempt.
+            destination (str): the domain in question
+        """
+
         msg = "Not retrying server %s." % (destination,)
         super(NotRetryingDestination, self).__init__(msg)