make \OC\Files\Node\Node implement the FileInfo interface

Instead of extending I opted for composition which allows lazy loading the fileinfo.

cc @DeepDiver1975 @PVince81 
Caching is good, I hope it will not cause side-effects if app devs are expecting the info to be up to date.
How to go about clearing this ? I suppose the API user should simply call root->get() again ?