import: `--jobs` option

[karajan1001]

dvc import Provides an easy way to reuse files or directories tracked in any DVC repository. This external tracked data might be stored in a remote DVC repository. In this situation --job which controls the parallelism level for DVC to download data from remote storage would be a useful option.

[karajan1001]

@efiop
Excuse me,
What is the difference between BaseTree.download and RepoTree.get_dir_hash?
In the first one, jobs is read from a config file while in the second, it was passed as a parameter.

And in
https://github.com/iterative/dvc/blob/6d745eb4f0fc4578e3abea25a9becaf2c9da1612/dvc/dependency/repo.py#L71-L80
This function inherited from the first one calls the second. Is it OK to add a jobs param to the first function which is now redundant?

[efiop]

@karajan1001 Usually for download the parallelization happens before calling it, e.g. in Remote.push/pull/status. For get_dir_hash, depending on how it is configured, it might need to pull the whole directory with all the files, and that's where the jobs is used. So considering that dvc is not splitting files into blocks and downloading whole files, --jobs for import only makes sense for directories.