Don't drop more than three consecutive packets

[larseggert]

I picked three, because if you loose the three initial ones in row, your PTO goes 1->2->4, meaning you waste 7 seconds, and many stacks use very short idle timeouts when they run.

I can look into making the limit configurable.

[janaiyengar]

I can see the value in this, especially since the RNG does this regression to the mean thing where consecutive drops happen occasionally and are annoying. That said, I've found it to be also useful in exposing issues -- stacks running with super low idle timeouts for instance sounds like a stack issue.

I'm concerned that if we never see consecutive drops of > 3 runs, that's going to hide particular classes of bugs.

[larseggert]

I'm fine with not merging this, but then we need to make stack images use some sane idle timeout.