Ingress/egress filter alarms

[dpino]

This PR implements the following alarms:

• Ingress filter v4 rejects over N packets/s
• Ingress filter v6 rejects over N packets/s
• Egress filter v6 rejects over N packets/s
• Egress filter v6 rejects over N packets/s

Partially fixes #965

[dpino]

To test it, I manually increased 'rxdrops' counter in pcap_filter:push and the limit was reached and the alarms triggered:

Excerpt of alarms get-state:

    alarm {
      alarm-type-id filtered-packets;
      alarm-type-qualifier egress-v6;
      resource 18482;
      alarm-text "More than 1,000,000 packets filtered per second";
      is-cleared false;
      last-changed 2017-09-29T15:08:10Z;
      perceived-severity warning;
      status-change {
        time 2017-09-29T15:08:10Z;
        alarm-text "More than 1,000,000 packets filtered per second";
        perceived-severity warning;
      }
      time-created 2017-09-29T15:08:10Z;
    }
    last-changed 2017-09-29T15:08:10Z;
    number-of-alarms 4;
  }
  summary {
    alarm-summary {
      cleared 0;
      cleared-closed 0;
      cleared-not-closed 0;
      not-cleared-closed 0;
      not-cleared-not-closed 4;
      total 4;
    }

[wingo]

LGTM with one wee question

[wingo]

Is it OK if this is nil?

[dpino]

Likely not, better to set to empty string if nil.

[dpino]

alarm-type-quailifier is not mandatory either in the definition of alarm-types and alarms, so I think it's fine if it's nil.