Remove memory/ ram store

[s1monw]

The RAM store is discuraged for production usage anyway and
we don't test it in our randomized infrastructure. This commit
removes it for 2.0

[jpountz]

LGTM, also nice to move a couple more methods to Paths instead of Files

[kimchy]

+1 on removing it, but lets give people more time to comment if possible

[dakrone]

+1, it's still possible to use ram disks if people absolutely need them and removing them reduces complexity

[clintongormley]

+1

[djschny]

This was a great benefit for unit testing when running an embedded version of elasticsearch as one did not have to clean up data afterwards. It's a shame to see it removed when it still has purpose. I'm trying to understand the stance here since it is clear what a memory/ram store does.

[clintongormley]

@djschny we've had issues in the past where there were bugs and inconsistencies in the ram store. If e provide it, we should also test it, otherwise we have no idea if it works correctly. Given that it is not recommended for production use, we decided to clear it out of our code base so that we can focus on the important stuff.

[djschny]

Thanks @clintongormley - missed the part where it was known to be buggy.