Downgrade from Aerospike Server 4.9 to 4.8

Background for possible need to downgrade: Upgrading to Aerospike Server version 4.9 can write some additional bits in preparation for future upgrades to handle XDR traffic. In some scenarios, specific steps are required when downgrading from 4.9 to 4.8 to remove these bits.

The downgrade steps from 4.9 to 4.8 must be followed only if the cluster is (or ever was) an active XDR destination with all nodes running 4.9.

Steps to downgrade an entire XDR destination cluster from Version 4.9 to Version 4.8

1. Before downgrading from 4.9 to 4.8, on each node, one at a time, shut down the node (quiescing it as necessary first), wipe the node's disks and bring it back up on the older version.
2. Migration then takes care of writing data in the correct storage format to the downgraded node.