Write out new-style statcache data.

Write out new-style statcache data.

Write out statcache-style data from the metadata logging module of lbs.
This will eventually replace the old statcache implementation, but is not
complete.  This new statcache data is not yet read in or used elsewhere.

In the new format, the data in the statcache file has the same format as
the data in the metadata log itself.  Each stanza with file information is
prefixed with a @@reference line that gives a reference to the location of
the metadata.  If the metadata has not changed, this will allow metadata
log data to be re-used between snapshots.