Sort-of-working statcache implementation.

Sort-of-working statcache implementation.

This will use stat information to determine when a file doesn't need to
be read again.  There are still some bits left to be implemented; in
particular, parsing of references needs to be fixed, since at the moment
checksums and ranges aren't supported, and any files using them will
have block lists corrupted by the stat cache.  (Fortunately, nothing
uses those reference forms yet for file contents.)