@Override
  protected synchronized void callTimeout(Call call) {
    // call sender
    calls.remove(call.id);
  }
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