Improved caching of sha1 values in a filesystem federation.

1.  Changed the extra properties caching to include the sha1 in the jcr:content properties.
2.  Changed the extra properties caching to include the file modification date in the jcr:content properties.
3.  Updated the extra properties cache retrieval to only use the cache value if the file modification date hasn't changed. (support updates outside of the federation)
4.  Updated the extra properties cache to omit the file size (which is easy enough to compute that it shouldn't be cached)