Save datetime along with outs md5 field

[prihoda]

Current datetime could be saved along with each output md5 field to make it easier to see which version of the file is the hash referencing.

cmd: wc -l input.txt > count.txt
deps: 
- md5: 3c57a863e8d24a77427a2b3dce171a4f
  path: input.txt
outs:
- cache: true
  updated: "2018-04-18 15:41:41"
  md5: f136ecf4fe2a555c1eefb51332f0f967
  path: count.txt

[efiop]

Hi @prihoda !

That is an interesting idea! I can definitely see how this could be useful. I will look into implementing it for 0.9.7. Though, if you wish to submit a proper patch, I will be happy merge it.

[dmpetrov]

We can also consider keeping file size in the dvc-file. It can help with the copy-semantic #612 - where you should think twice before running dvc checkout.

[villasv]

File sizes would be helpful. Once I had an empty output file and had to trace back the step in which it became empty through the pipeline and the commit that introduced the bug. Would have been pretty easy if I had that info was also tracked.

[dmpetrov]

A couple more things we can consider:

1. the data time could be used for the garbage collector - very natural way to reduce size. dvc gc --time month - clean everything not from the current branch and older than a month.
2. @villasv brought a good point about empty size output. Based on my experience in 99% it is a red flag. For a single output step - in 99.9% of cases an empty file is a sign of an error. We can consider printing a warning in this case.

[efiop]

Sorry, missed @villasv 's comment. It is indeed a great idea. Created a task for it: #677 .

@dmpetrov dvc gc --time month is a great idea as well. Created #678 . Need to think about particular details, e.g. maybe we also need to check if file is in the cloud before deleting local cache. I will look into it soon as well.

[efiop]

After a bit more thinking, it seems like by saving datetime we would be cluttering our dvcfiles(which should be hand-mergeable, thus they need to be as simple as possible) with the info that could be easily retrieved from git log, as you should generally commit your changes anyway.