JBIDE-17148 api enhancement for url transport utility class

General structure of the change is that:
  1) A bug was fixed where caches were always incorrect. (String was not url-decoded on load)
  2) Cache indexes have been moved to a standalone index file for each cache
  3) The cached file name now more closely resembles the url it is for, rather than cache-324232432.tmp
  4) A new method signature is added to public API class URLTransportUtility where a cache root can be passed in.