public Map<NodeType, Version> getTargetVersions() {
        return Collections.unmodifiableMap(db.readInfrastructureVersions());
    }
Multithread safe class to see and set target versions for infrastructure node types.
{@link InfrastructureProvisioner} maintainer will then allocate the nodes of given node type
with a wanted version equal to the given target version.

@author freva
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/maintenance/InfrastructureVersions.java