osd: use stronger hash function for mapping objects -> pgs

osd: use stronger hash function for mapping objects -> pgs

The old hash (from linux dcache) was very weak, such that
least sig bits may not change and you could get lots of
consecutive objects on the same osds (because lsbits of the
pg weren't changing).

This is Robert Jenkin's hash and is quite strong.  Public
domain.

Rev osd disk format, protocol, since we're totally changing
object placement here.