stop using strings for filepaths and start using File objects

Catching up on my email after a long hiatus i notice SOLR-1213 and it reminded me that the last time i was digging arround in the code i was frustrated by lots of places in Solr where "String" instances to store a file or directory paths and then manipulated with String operations. I would be nice to switch all of these member variables and and function params to use genuine File objects to improve the code base readibility, and reduce the likelyhood of OS variant bugs or nonsensical path operations.