private void addScannerLeaseBack(LeaseManager.Lease lease) {
    try {
      server.getLeaseManager().addLease(lease);
    } catch (LeaseStillHeldException e) {
      // should not happen as the scanner id is unique.
      throw new AssertionError(e);
    }
  }
The returned String is used as key doing look up of outstanding Scanners in this Servers'
this.scanners, the Map of outstanding scanners and their current state.
@param scannerId A scanner long id.
@return The long id as a String.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RSRpcServices.java