A format for an FS dump needs to be established. The format should be
independent of internal node ID type stuff. See Bill Tutt's model of the SVN for
an idea of the data elements and their relationships. Ideally, if the format
followed the models, then it would be (relatively) independent of internal concerns.
The dump/restore would be built as FS interface functions, and svnadmin
subcommands to invoke those functions.
Note that we need this format as an enabling feature to alter internal node IDs
and FS structure. Setting as P2 because we'll need this soon to do our planned
node ID conversion.