rbd: implement 'export-diff' and 'import-diff' commands

Export a diff of an image from a previous snapshot to a file (or stdout).

Import a diff and apply it to an image, and then create the ending
snapshot.

Signed-off-by: Sage Weil <sage@inktank.com>
rbd: add --io-pattern <seq|rand> option to bench-write

Write to random offsets instead of sequentially.

Signed-off-by: Sage Weil <sage@inktank.com>
allow import without startsnap
show progress even from stdin
better test