Improve CDP topology calculation performance

The current algorithm used to generate CDP links in the Enlinkd topology operates in O(n^2) where n is the number of CDP links. On a system with 1500 nodes and 35000+ CDP links, the function call takes > 5 minutes.

The relevant code block is here:

https://github.com/OpenNMS/opennms/blob/a1f3b44b95f60692df5b8a490cee99aa4d08fb6b/features/topology-map/plugins/org.opennms.features.topology.plugins.topo.linkd/src/main/java/org/opennms/features/topology/plugins/topo/linkd/internal/LinkdTopologyProvider.java#L402

Reviewing the algorithm, I think we can make this work in O( n ) by indexing the links in a first pass, and then matching them in a subsequent pass.