import-url: lock by default

[efiop]

To make it symmetrical to dvc import command #2218 . For updating, we could introduce dvc update-url

[Suor]

I don't think we need update-url. Why not use dvc update to update that?

[efiop]

@Suor to have an ability to update ones or the others. Though, for that one could specify particular .dvc files, so the usefulness is disputable :)

[efiop]

Had a discussion and decided to go with dvc update for urls too.

[efiop]

Need to make import-url lock by default and make dvc update work on import-urls.