Provide QueryAcceptors to be declared in configuration

  private List<QueryAcceptor> queryAcceptors = new ArrayList<QueryAcceptor>();

  private void accept(String query, Configuration conf, SubmitOp submitOp) throws LensException {
    // run through all the query acceptors, and throw Exception if any of them
    // return false
    for (QueryAcceptor acceptor : queryAcceptors) {
      String cause = "";
      String rejectionCause = acceptor.accept(query, conf, submitOp);
      if (rejectionCause != null) {
        getEventService().notifyEvent(new QueryRejected(System.currentTimeMillis(), query, rejectionCause, null));
        throw new LensException("Query not accepted because " + cause);
      }
    }
    getEventService().notifyEvent(new QueryAccepted(System.currentTimeMillis(), null, query, null));
  }