Change the label from InterfaceAudience.Private to InterfaceAudience.LimitedPrivate for org.apache.hadoop.hbase.Server.java

Currently the class org.apache.hadoop.hbase.Server.java is marked InterfaceAudience.Private.

This prevents in getting shared zookeeper watcher (ZKWatcher) instances from org.apache.hadoop.hbase.regionserver.RegionServerServices.java (extends org.apache.hadoop.hbase.Server.java) using the method getZooKeeper().

This creates a problem in writing custom Coprocessors because now we don't have a shared ZKWatcher instance.

The proposed solution is to use the InterfaceAudience.LimitedPrivate.