Alluxio integration

[gm-spacagna]

I would encourage to leverage Alluxio technology, in particular the fuse mount feature, for synchronizing local file system with remote data repositories such as S3.
Let's suppose the data is very large and you do not want to fully copy it locally, you can store the data in S3 and read smartly from the Alluxio tiered cache via the local file system.
lazydata should then only check consistency of files, store and track the metadata attributes into git.

[rstojnic]

I'm not very familiar with Alluxio, would you be able to provide a pull request?