checkout: print reminder of other link types on default link cache

[pared]

Related to #1257
After mergin #1841 we make reflink and copy default link types.
That might slow process of checking out for bigger files. Implement mechanism to detect prolonged checkout time and print some kind of information that other link types might speed process up. Also, there should probably be easy way to turn this message off, if someone deliberately leaves defaults.

[shcheklein]

We should also add the message when we move files to the cache initially - dvc add, dvc run, dvc import...

[pared]

Ok, can I create new issue for that or would you rather see that in current one?

[shcheklein]

@pared I would split if the patch for the first part is almost ready

[pared]

Ok, ill just rename config options so that they match all situations, to prevent potential backward incompatibility.

[efiop]

slow_link_warning is the one that we need. It applies to add, checkout, etc. And it actually might answer the question of how this warning should be handled in general 🙂

[pared]

Example:
https://asciinema.org/a/2XsHkAUcUfx1aHfmUV7atnpoJ