New module XrdFileCache for disk based file cache

This pull request contains the following changes:

-  XrdFileCache : implementation of disk based file cache.

- packaging : add libXrdFileCache.so to list of server libs

- XrdPosix : src/XrdPosix/XrdPosixFile.cc in Finalize() -- Stat() the file before creation of the cache objects as they need to know the size of the file.