The Threaded Runtime is Broken!!!1!

[acw]

The threaded runtime appears to have trouble switching from high-usage threads. This behavior appears at its worst in Hans, in which traffic piles up in one layer without a context switch relieving the pressure on the intermediary channels. It's not clear if this is a problem in which we're not doing context switches across capabilities when we have a limited number of CPUs, or if there's a fundamental problem in how we're catching thread change signals from outside.

Actually, the fact that profiling works irregularly in the threaded runtime may suggest the latter.