rgw: Disentangle init_sync_status from RemoteBucketManager

rgw: Disentangle init_sync_status from RemoteBucketManager

RGWRemoteBucketManager's current design isn't really compatible with
what we need for bucket sync run to work as the number of shards
changes from run to run.

We can make a smaller 'hold information common to all three
operations' class and simplify things a bit.

We also need to fetch `rgw_bucket_index_marker_info` and supply it to
`InitBucketFullSyncStatusCR` to ensure we have the correct generation
and shard count.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>