ZOOKEEPER-2795: Change log level for "ZKShutdownHandler is not registered" error message

`ZKShutdownHandler` may not be registered if the user creates a `ZooKeeperServer` object outside of `ZooKeeperServerMain.runFromConfig`.

We should change the log level of the message that is printed on state changes of `ZooKeeperServer` when `ZKShutdownHandler` is missing to something lower.

Author: Abraham Fine <afine@apache.org>

Reviewers: phunt@apache.org

Closes #273 from afine/ZOOKEEPER-2795

Change-Id: I8b048f6b79870064325925a6ff52f7d1f3e5e1da
(cherry picked from commit 86e35b5ae4e846e84dcc3ec08f724af7aa4ec2aa)
Signed-off-by: Patrick Hunt <phunt@apache.org>

src/java/main/org/apache/zookeeper/server/ZooKeeperServer.java

@@ -506,7 +506,7 @@ public class ZooKeeperServer implements SessionExpirer, ServerStats.Provider {
         if (zkShutdownHandler != null) {
             zkShutdownHandler.handle(state);
         } else {
-            LOG.error("ZKShutdownHandler is not registered, so ZooKeeper server "
+            LOG.debug("ZKShutdownHandler is not registered, so ZooKeeper server "
                     + "won't take any action on ERROR or SHUTDOWN server state changes");
         }
     }