Add the --rebuild-statcache option.

Add an option which forces cumulus to re-read all files, even if they seem
not to have changed.

This has two main purposes: first, it helps to rebuild the contents of the
statcache from data actually on disk, and as such is useful for adding
object size annotations to files which haven't changed (ordinarily, the
straight re-use of data from the statcache prevents this).

Second, it can be used to detect some simple forms of data corruption; if a
file has not changed (according to stat information) but the checksum
doesn't match the old value in the statcache file, a warning is printed.