Add getJournalSyncerStatus jmx metrics, to Indicates whether the JournalSyncer thread for the every namespace has enter working state.

The JornalNode JVM process is not immediately in a normal state until the JournalSyncer thread is started.
For some management platforms such as Ambari rolling restart JournalNode, we need a jmx metric to determine whether the JournalSyncer thread is started and enter working state for current namespace before restarting the next JournalNode. Otherwise, restart too quickly and more than half of JournalNodes will be out of order, causing the NameNode to die.

When i add it , the effect is as follows: