CPL: Robustly recover from failures during restart-file writing.

It's possible for a crash that occurs during restart writing to lead to
inconsistent or incomplete rpointer files. While we can't salvage the restart
files in general, we can at least provide a consistent set of rpointer files --
namely, the previous ones -- for the next restart.

The rpointer manager provides this capability by copying all rpointer.X files to
rpointer.X.prev before components write restart files, then removing
rpointer.X.prev files when all components are done.

If a crash occurs midway through, on restart, the consistent rpointer.X.prev
files will be used.