Resume downloading on import

[efiop]

Considering that big files aren't something unusual in dvc scenarios, we should support resuming import.

[efiop]

We used to have support for this in the old dvc, but it got lost during the redesign. Need to bring back this functionality. Maybe even with continue==True by default.

We could start by implementing this for HTTP module, since we have an old implementation to serve as a guide of some sorts.