Revert addition of rcs_bucket_v2 record, use moss_bucket_v1 (as 1.4)

Because bucket records are used in user recoreds stored in Riak,
introducing its new version requires rolling upgrade of all riak-cs
nodes with cs_version trick.

rcs_bucket_v2 had the additional field "manifest_bag", which is used
only in storage calculation. In storage calculation, bucket objects
fetch are added and bag_id's are now extracted from them.  This adds
some GET calls to Riak but they are much less costly compared to
map-reduce.