Better handling for blocking tasks

[morgante]

When we pick up a slow file (primarily because it invokes JavaScript callback or other large overhead) we want this to only block that thread.

It's not a huge problem to block one thread at a time since the ratio if slow work to fast work is usually fine. The problem is when we allow fast work to queue up on that same thread.

[morgante]

In #546 I investigated and if some tasks are much slower, it looks like spawn is a meaningful improvement over par_iter.