Fix RWS's error 400 when starting ProxyService with more than 0 subs

Sometimes RWS will get stuck and produce warnings about "invalid data".
Apparently, this can be caused by the fact that ProxyService starts by
sending the submissions/subchanges related data first, and then it will
proceed to the contests/teams/users related data. Now, if there are
already more than 0 submissions, this means that ProxyService will send
data about these submissions even though it has never initialized the
rankings.

This commit enqueues the initialization operation and *then* starts the
sweeper.

cms/service/ProxyService.py

@@ -262,11 +262,12 @@ def __init__(self, shard, contest_id):
         self.rankings = list()
         for ranking in config.rankings:
             self.add_executor(ProxyExecutor(ranking.encode('utf-8')))
-        self.start_sweeper(347.0)
 
-        # Send some initial data to rankings.
+        # Enqueue the dispatch of some initial data to rankings.
         self.initialize()
 
+        self.start_sweeper(347.0)
+
     def _missing_operations(self):
         """Return a generator of data to be sent to the rankings..