Ensure that wait_for_sync always does a new sync

If a sync is in progress, wait for that to complete.

Then issue the requested *new* sync.

Probably only needed if fullsync, as the one in progress may be a
'dirty' only sync and fullsync would be satisfied by the partial sync
without it, which is bad.

confluent_server/confluent/config/configmanager.py

@@ -1906,6 +1906,8 @@ class ConfigManager(object):
 
     @classmethod
     def wait_for_sync(cls, fullsync=False):
+        if cls._cfgwriter is not None:
+            cls._cfgwriter.join()
         cls._bg_sync_to_file(fullsync)
         if cls._cfgwriter is not None:
             cls._cfgwriter.join()