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

pcap.go

@@ -40,9 +40,6 @@ func newPcapIO(ifName string, promisc bool, snaplen int, bufSz int) (handle *Pca
 	if err = inactive.SetSnapLen(snaplen); err != nil {
 		return
 	}
-	if err = inactive.SetTimeout(-1); err != nil {
-		return
-	}
 	if err = inactive.SetImmediateMode(true); err != nil {
 		return
 	}