Add script to repair invalid gc manifests

This PR addresses the remaining part of https://github.com/basho/riak_cs/issues/827 .
( #964 is another part)

This PR adds a script to repair invalid state manifests in GC bucket when
- manifest in GC bucket is `active` and
- corresponding actual manifest (in `0o:*` bucket) is `pending_delete`, `scheduled_delete` or `notfound`

 
Don't forget to update this before code freeze!