rgw: address bug where object puts could write to decommissioned shard

rgw: address bug where object puts could write to decommissioned shard

Addresses an apparent bug where some object writes that initiate
around the time that resharding completes are written to the old
bucket index shard rather than the new one.

Other write operations to a bucket index shard now also assert their
existance so as not to re-create old bucket index shards.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>