In particular, one thing that we can't (properly) do before this commit
is consuming an infinite input stream. For example:
```
yes | grep y | head -n10
```
will give 10 "y"s in most shells, but blocks indefinitely in nu. This PR
resolves that by doing blocking I/O in threads, and reducing the `await`
calls we currently have in our pipeline code.
7304d06c0b