rados: make 'tmap dump' gracefully handle non-tmap data

Signed-off-by: Sage Weil <sage@inktank.com>
rados: add 'tmap-to-omap' command

Explicitly convert tmap object data to omap keys.  Removes the old tmap
content at the same time.

Signed-off-by: Sage Weil <sage@inktank.com>