code: private long getLastClosedTimestamp(SocketDestination socketDestination) {
          Long lastClosedTimestamp = lastClosedTimestamps.get(socketDestination);
          return lastClosedTimestamp != null ? lastClosedTimestamp.longValue() : 0;
      }
docs: Returns the nanosecond-based timestamp of when this socket destination
  was last closed. SocketDestination objects can be closed when their node
  is marked as unavailable if the node goes down (temporarily or
  otherwise). This timestamp is used to determine when sockets related to
  the SocketDestination should be closed.
  
  <p/>
  
  This value starts off as 0 and is updated via setLastClosedTimestamp each
  time the node is marked as unavailable.
  
  @return Nanosecond-based timestamp of last close