Server restart might mangle an internal database

[wtlgo]

Describe the bug
If the server restarts or crashes before the plugin can add all heads to the database it never tries to finish the download. Since the plugin does not specifically check the integrity of added data and instead just checks that there is some data in the DB, and the server admin has no command or any other means to resume the download, it becomes very inconvenient.

To Reproduce

1. Clear the "plugins" folder.
2. Add the plugin.
3. Start the server.
4. Restart the server before it says "Finished inserting all heads into the data file!"
5. Suffer

Expected behavior
It would be nice if there was a way to resume the download process, or plugin checked the integrity of downloaded data and resumed it automatically.