Tor sometimes stalls traffic at 2s intervals (6s most common)

Dennis Jackson found some very suspicious performance issues while scanning the Tor network: https://lists.torproject.org/pipermail/tor-scaling/2019-July/000063.html

The "6s delay" mystery there looks like some kind of relay bug. There are smaller bumps for delays at 2s multiples.. Cause is still unknown. We should track this one down and fix it.