Experimental capture optimization (pcap & af-packet)

This PR reduces calls to gettimeofday. This is currently done for each packet. The calls are unnecessary as we already know the time.

Prscript:
- PR build: https://buildbot.openinfosecfoundation.org/builders/inliniac/builds/33
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/inliniac-pcap/builds/33

@pevma can you test af-packet and report if you see any perf difference?
cc: @regit, @glongo