This method is invoked during DN startup when volumes are scanned to
build up the volumeMap.
Given two replicas, decide which one to keep. The preference is as
1. Prefer the replica with the higher generation stamp.
2. If generation stamps are equal, prefer the replica with the
larger on-disk length.
3. If on-disk length is the same, prefer the replica on persistent
4. All other factors being equal, keep replica1.
The other replica is removed from the volumeMap and is deleted from
its storage volume.