cls_rgw, rgw: bucket index logs modifications

Add a log to the bucket index.

This commit also ties the "epoch" version that is kept
per index entry to the relevant ceph pool id. This fixes
the dependency on the specific pool where the object was
created, and makes it possible to copy rgw data pools.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>