evolve: add a debugobsstorestat command This commands print multiple some data about the content of the obsstore. This will be usefull to provide some metric and insight for obsolescence marker exchange.

evolve: extend obsstore object to use prune parent information Now that we have the information, we can use it to build a mapping to access all marker that prune a children of a node. This is similar to what we do for successors and precursors. The performance impact is a few tens of second on my mercurial clone. Experimenting is worth the trouble.