A host callback marks where it runs on Node's stack, and a call into
Node.js from a fiber starts below that mark. Runtime code the callback
runs may go deeper on Node's stack before it starts or resumes a fiber,
and those frames stay live while the fiber runs: a call into Node.js
from the fiber overwrote them. scr_switch now reports where it leaves
Node's stack, and the mark moves down to it for the rest of the
callback.
6823 runs a timer callback 200 frames deep that starts an async
function calling into Node.js before its first await.
Chris Tate
1a2eb52016