Move dns names from endpoints to report level

[bboreham]

In large reports we can see many repeats of the same name lookup:

      ";10.244.252.166;42230": {
        "id": ";10.244.252.166;42230",
        "topology": "endpoint",
        "counters": null,
        "sets": {
          "reverse_dns_names": [
            "ip-10-244-252-166.ec2.internal"
          ]
        },
...
      ";10.244.252.166;42170": {
        "id": ";10.244.252.166;42170",
        "topology": "endpoint",
        "counters": null,
        "sets": {
          "reverse_dns_names": [
            "ip-10-244-252-166.ec2.internal"
          ]
        },
...
      ";10.244.252.166;42200": {
        "id": ";10.244.252.166;42200",
        "topology": "endpoint",
        "counters": null,
        "sets": {
          "reverse_dns_names": [
            "ip-10-244-252-166.ec2.internal"
          ]
        },
...

so we could have a structure up at Report level that mapped them once, then just look this up when we need it.
For backwards-compatibility we could create this structure in Report.Upgrade()

Relates to #1201