Show nexthop MAC in top and yang model

[dpino]

This PR adds two extra fields to snabb-softwire-v2.yang: next-hop-macaddr-v4 and next-hop-macaddr-v6. Both values can be queried using snabb config get-state and using snabb top.

Some thoughts about the PR:

• Perhaps is worth to set an interval of time for updating the next hop mac address values.
• Counters are always of uint64_t type. That's fine for MAC addresses, however in the YANG model I wanted the mac address fields to be of type yang:mac-address. This required me to do modify lib.yang.value's macaddr type and handle the case in tostring where val might be an uint64_t value (uint8_t is expected). I didn't find a more elegant way of implementing this.

Some examples:

• Snabb top:

  workers:
    24764:
      pid: 24764
      apps:
        lwaftr:
          next-hop-macaddr-v4: 02:99:99:99:99:99                                        next-hop-macaddr-v6: 02:99:99:99:99:99

• Snabb config:

$ sudo ./snabb config get-state --schema=snabb-softwire-v2 diego / | grep mac
      next-hop-macaddr-v4 02:99:99:99:99:99;
      next-hop-macaddr-v6 02:99:99:99:99:99;