--concurrency option seems to cause non-deterministic I/O errors

[hkeeler]

I have not been able to confirm this bug, but through a fair amount of observation, it seems errors crop up when --concurrency or -c is set to more that 1. They are generally I/O errors, such as FTP download and file unzipping, and the errors happen for different files and FTP sites on subsequent runs of the process, but at least one usually occurs on a given run of the loader. I have attempted a couple runs of the loader with -c 1, and no errors occurred.

Also of note, I've only seen this while running in a Docker container. I don't see how that would cause this, but seemed worth mentioning.