Only update mPipe stats occasionally around the packet loop.

Check for termination or stats update only once every 10,000 times
around the mPipe packet processing loop, to reduce locking.

Passes prscript:
https://buildbot.suricata-ids.org/builders/ken-tilera/builds/120
https://buildbot.suricata-ids.org/builders/ken-tilera-pcap/builds/54