pcap: 32bit counters can wrap-around

pcap: 32bit counters can wrap-around

Fixes issue 2845.

pcap_stats is based on 32bit counters and given a big enough throughput
will overflow them. This was reported by people using Myricom cards which
should only be a happenstance. The problem exists for all pcap-based
interfaces.

Let's use internal 64bit counters that drag along the pcap_stats and
handle pcap_stats wrap-around as we update the 64bit stats "often enough"
before the pcap_stats can wrap around twice.