Added HxCheckpoints, optional setting to store checkpoints on a separate
volume from depots. Default is same value as HxDepots.
This changes makes the SPD more adaptable to customers, especially using
NFS, that prefer to have separate mounts for checkpoints vs. those
for archive files. The original SDP design was to have everything that
needed to be backed up (i.e. metadata checkpoints and the versioned file
tree) on a single volume. This is still the preferred mode, but allowing
checkpoints and archives to be separate is useful for some.