gluon-mesh-batman-adv: add "routes" to respondd neighbours

[lemoer]

This commit adds counters about the number of imported and selected routes for each neighbour. The following stanza

"routes": {
    "imported": 171,
    "selected": 120
},

means, that 171 routes have been learned from this neighbour, but only 120 of them are selected. (For the other 51 routes, the node prefers other neighbours that also provide routes to the same destination).

These fields are added to the gluon to identify route flaps. This should make it easier to troubleshoot connectivity problems that only occur when the route swaps to a weak link.

After the new field "routes" is added, the neighbours response looks like this:

{
  "wifi": [
  ],
  "batadv": {
    "72:4c:e2:db:6f:37": {
      "neighbours": {
        "88:e6:40:20:90:10": {
          "routes": {
            "imported": 171,
            "selected": 171
          },
          "best": true,
          "lastseen": 0.95599999999999996,
          "tq": 247
        }
      }
    }
  },
  "node_id": "525400123457"
}