remove dumped data after load finished

[csuzhangxc]

Feature Request

Is your feature request related to a problem? Please describe:

the dumped data was not removed even after all of them loaded successfully.

Describe the feature you'd like:

• remove dumped data automatically after loaded.
• add a new config item to keep the dumpled data if needed.

[lance6716]

I'm going to implement automatic deletion by periodically check whether offset in checkpoint equals to file size. Still learning code.