Add LRU cleanup for generic repositories

[AJIOB]

Implements suggestion from #130

[allburov]

I don't think we should combine LRU and sized rule here.
Almost all rules right now works on "number of files" idea - so for LRU it'd be something like

- rule: DeleteLeastRecentlyUsedFiles
  keep: 10

where 10 - number of artifacts.

Imagine the case when you setup 512MB limit with regular size for each artifact for 10Mb.
But accidentally, you put some large zip/deb/msi in the repo, like 640Mb.
The rule with Mb will remove all other artifacts, but will keep only the last one.