bgpd: Show prefix-related stats per neighbor

bgpd: Show prefix-related stats per neighbor

E.g.:

```
  Prefix statistics:
    Inbound filtered: 0
    AS-PATH loop: 0
    Originator loop: 0
    Cluster loop: 0
    Invalid next-hop: 0
    Withdrawn: 0
    Attributes discarded: 3
```

JSON:

```
    "prefixStats":{
      "inboundFiltered":0,
      "aspathLoop":0,
      "originatorLoop":0,
      "clusterLoop":0,
      "invalidNextHop":0,
      "withdrawn":0,
      "attributesDiscarded":3
    },
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>