Add exception handler for lease thread

In a prod cluster, a region server exited for some important
threads were not alive. After excluding other threads from the log, we doubted the lease thread was the root.

So we need to add an exception handler to the lease thread to debug why it exited in future.

2015-06-29,12:46:09,222 INFO org.apache.hadoop.hbase.regionserver.HRegionServer: STOPPED: One or more threads are no longer alive – stop
2015-06-29,12:46:09,223 INFO org.apache.hadoop.ipc.HBaseServer: Stopping server on 21600
...
2015-06-29,12:46:09,330 INFO org.apache.hadoop.hbase.regionserver.LogRoller: LogRoller exiting.
2015-06-29,12:46:09,330 INFO org.apache.hadoop.hbase.regionserver.MemStoreFlusher: Thread-37 exiting
2015-06-29,12:46:09,330 INFO org.apache.hadoop.hbase.regionserver.HRegionServer$CompactionChecker: regionserver21600.compactionChecker exiting
2015-06-29,12:46:12,403 INFO org.apache.hadoop.hbase.regionserver.HRegionServer$PeriodicMemstoreFlusher: regionserver21600.periodicFlusher exiting