[metadata-vs-api] resuming of font file downloads

[felipesanches]

Originally posted at: https://github.com/googlefonts/fontbakery/issues/659

@felipesanches said:

• [ ] partially downloaded files should resume instead of starting from scratch.
• [ ] too old files should be re-downloaded even if cache is used (i.e. invalidate download cache after a certain amount of time (how much time?))