Save xml files (from a cscan run) in the same cache directory that is used for filetables.
But still use a cached xml file from the data directory if a suitable one be there.
When building a filetable, remember the path in which a filetable is to be cached.  Remember it as the _cache_path attribute of the filetable.
Add capability to remember a cache path as an attribute of a basic_filetable, and a method to report it.