Double pre-buffering for better performance with very large pcaps

[synfinatic]

Use the TCPivo method of dual memory buffers to prefetch packets from disk. Locking penalties are reduced by having two buffers which are at any given time a write buffer (from disk) or a read buffer (to network). Only really useful if you're using a really large pcap file and you can't use the cache file to store it completely in memory IMHO.

Not as important now that we have a full cache option in 3.0. Still worth doing for large pcaps (larger then free RAM) or when you're not looping.