Add rejection-processable InvokeCallback . fix #178

[cytnju]

add rejection-processable InvokeCallback to provide flexible options when user executor rejected task.

public interface RejectionProcessableInvokeCallback extends InvokeCallback {

    /**
     * when user executor rejected the {@link com.alipay.remoting.rpc.RpcInvokeCallbackListener.CallbackTask},
     * bolt will handle the rejected task according to this {@link RejectedExecutionPolicy}
     * @return rejectedExecution Policy
     * @see com.alipay.remoting.rpc.RpcInvokeCallbackListener#onResponse(InvokeFuture)
     */
    RejectedExecutionPolicy rejectedExecutionPolicy();
}

public enum RejectedExecutionPolicy {
    /* discard the callback task */
    DISCARD,
    /* caller runs the callback in IO-thread */
    CALLER_RUNS,
    /* caller handle the task with exception strategy user provided */
    CALLER_HANDLE_EXCEPTION
}