Bucket index sharding - metadata change, bucket initialization

I would like to split the bucket index sharding effort into a couple of patches (considering the scope of the change) with multiple commits (which is easier to review).

This is the first one with following changes:
  1. Add a new field to bucket info to indicate the number of shards.
  2. Implement bucket initialization logic.

Testing:
  Test done making sure all the shards are created properly.

Signed-off-by: Guang Yang (yguang@yahoo-inc.com)