Fix security key initialization race condition

When initializing security key, a background thread may occur.  Sometimes,
the system would go to daemonize while that thread was still running, and
the whole system could exit.  Leading to incomplete write to globals as well
as leaving the daemon looking at the data copied over from pre-fork and
seeing the last state of that thread forever frozen.  Make sure the background
threads are fully done prior to exiting.

confluent_server/confluent/config/configmanager.py

@@ -1238,10 +1238,14 @@ class ConfigManager(object):
             pass
 
     @classmethod
-    def shutdown(cls):
+    def wait_for_sync(cls):
         cls._bg_sync_to_file()
         if cls._cfgwriter is not None:
             cls._cfgwriter.join()
+
+    @classmethod
+    def shutdown(cls):
+        cls.wait_for_sync()
         sys.exit(0)
 
     @classmethod

confluent_server/confluent/main.py

@@ -145,6 +145,9 @@ def _initsecurity(config):
         with open(keyfile, 'r') as keyhandle:
             key = keyhandle.read()
         configmanager.init_masterkey(key)
+        # We don't want to os._exit() until sync finishes from
+        # init above
+        configmanager.ConfigManager.wait_for_sync()
 
 
 def run():