If we have the ability to lazy load Promise values, i.e. if we have a
debug channel, then we should always use it for Promises that aren't
already resolved and instrumented.
There's little downside to this since they're async anyway.
This also lets us avoid adding `.then()` listeners too early. E.g. if
adding the listener would have side-effect. This avoids covering up
"unhandled rejection" errors. Since if we listen to a promise eagerly,
including reject listeners, we'd have marked that Promise's rejection as
handled where as maybe it wouldn't have been otherwise.
In this mode we can also indefinitely wait for the Promise to resolve
instead of just waiting a microtask for it to resolve.
e6dc25daea