LRU auto-strategy for GC

[dmpetrov]

It would be convenient to set up a strategy like LRU instead of removing everything or by time #678.

Use cases:

1. dvc gc --stragegy LRU 10Gb - release 10Gb from cache
2. dvc config Core.LRU_Limit 20Gb && dvc run -d input/ -o output.tsv Rscript - it will GC automaticaly as a result of dvc run if cache size if more then 20Gb

This should work on a shared cache as well for multiple users.

It looks like this appproach will require a special state file in a cache directory to support LRU strategy and some of the DVC commands will modify the file: dvc run, dvc repro etc.

[erdnaavlis]

If I may step in, it would be cool to have the option for:

3. dvc gc --stragegy LRU 10Gb - release files from cache until it is equal to, or less than 10GB.

And raise an error or a warning if it needs to release files required for current working commit/branch.

[dmpetrov]

@andrethrill you are right! It is better to specify a limit for cache, then amount to release.