Move all the wrapper functions for distributor.fire

Move the functions inside the distributor and import them
where needed. This reduces duplication and makes it possible
for flake8 to detect when the functions aren't used in a
given file.

synapse/util/distributor.py

@@ -15,7 +15,9 @@
 
 from twisted.internet import defer
 
-from synapse.util.logcontext import PreserveLoggingContext
+from synapse.util.logcontext import (
+    PreserveLoggingContext, preserve_context_over_fn
+)
 
 from synapse.util import unwrapFirstError
 
@@ -25,6 +27,24 @@ import logging
 logger = logging.getLogger(__name__)
 
 
+def registered_user(distributor, user):
+    return distributor.fire("registered_user", user)
+
+
+def user_left_room(distributor, user, room_id):
+    return preserve_context_over_fn(
+        distributor.fire,
+        "user_left_room", user=user, room_id=room_id
+    )
+
+
+def user_joined_room(distributor, user, room_id):
+    return preserve_context_over_fn(
+        distributor.fire,
+        "user_joined_room", user=user, room_id=room_id
+    )
+
+
 class Distributor(object):
     """A central dispatch point for loosely-connected pieces of code to
     register, observe, and fire signals.