[DNM] Prototype review for rgw bucket index sharding

Hi Yehuda,
Can you help to take a look at this code change (at high level) to support bucket index sharding, the work has not completed yet (basic put/listing test passed, but not for all bucket related operations), so that I take this as a very initial draft review for the prototype. If things are good, I will continue the effort and hopefully send out a complete review next week.

In terms of *ver* field of bucket index header, as it involves rgw agent that I haven't looked into yet. I will include that part as well.

Thanks very much.