Dvc: core.checksum_jobs doesn't work for remotes

[efiop]

https://github.com/iterative/dvc/issues/2891#issuecomment-562812250

[Suor]

The proper fix might require implementing something like config inheritance:

• look at remote section first
• look at core second
• use default

For particular options like checksum_jobs.

This will require making config smarter most probably. So it's a research now.

[JIoJIaJIu]

I'm starting to work on the issue