Improve node cache in flow document enrichment

In the flow DocumentEnricher, we current enrich the documents with node information related to the export and src/dst addresses of the flow: https://github.com/OpenNMS/opennms/blob/92e4031c75cbfff6c41e42dabe074ba362e9528e/features/flows/elastic/src/main/java/org/opennms/netmgt/flows/elastic/DocumentEnricher.java#L111

For exporters, we expect to find a matching node in most if not all cases.
For src/dst addresses, we do not expect to find a node in most cases.

Exporters can be expected to have a cardinality of about 1000, whereas src/dst addresses are expected to have a much higher cardinality in the order or hundreds of thousands.

With the current structure, this would mean that we need a cache sized with cardinality(exporters) + cardinality(src&dst) addresses in order to avoid constant evictions.

As an alternate scheme, I propose the following:

• Always query the InterfaceToNodeCache to retrieve the node id
  • The complete set of interface to node mappings is already in memory, so this will not require any DB hits
• Maintain a cache from node id to NodeDocument that is only used when an node id is found from the previous lookup

Now, we only need to maintain a cache that is in the order of nodes in the system, rather than unique src/dst addresses.