Checkpointing v1 in diagrams

[Julian Foad]

Here are a couple of diagrams to show how Checkpointing v1 behaves so far.
1. Shelve, Unshelve

Shows a difference between how Shelving-v1 worked and how it works in 
the new sheving-and-checkpointing design. Note that Unshelve no longer 
deletes the shelf. Shelving again would create a second version "foo 2".

2. Checkpoint, Revert, Restore

Shows how checkpointing and restoring works. Note that a 'revert' is 
needed before restoring.

(SVG format, drawn in Inkscape.)

- Julian