Only write out metadata files and the metadata-providers.xml file if something has changed

Right now, the ShibUI has a property which controls how often individual metadata files and the metadata-prpviders.xml file it manages are written out to disk. But this currently rewrites all files it is managing (that are "enabled"), whether or not any info that ends up in that file has changed. As long as one has a "smart file transfer" process – e.g. rsync or git – in place that only transfers/updates based on actual changes, that isn't a big deal.

But in case one does not, and to provide the "least surprising/most efficient" behavior, it would be best to only rewrite files that actually have changed. Enhance that file writing process to do so.