don't set pcap timeout, to prevent cpu churn
pcap timeouts are a mystery. Setting the timeout to -1 used to reduce
latency, but that was before we started setting immediate mode. And it
turns out that with that timeout, pcap chews through a lot of CPU
polling its file descriptor, checking timers, etc.

With no timeout set an idle weave is completely idle - no system calls
at all.

Fixes #80