@Override
  public synchronized void shutdown() {
    closed = true;
    if (callSender != null) {
      callSender.interrupt();
    }
    closeConn(new IOException("connection to " + remoteId.getAddress() + " closed"));
  }
Initiates a call by sending the parameter to the remote server. Note: this is not called from
the Connection thread, but by other threads.
@see #readResponse()
hbase-client/src/main/java/org/apache/hadoop/hbase/ipc/BlockingRpcConnection.java