Use threads to avoid blocking reads/writes in externals. (#1440)

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.

tests/shell/pipeline/commands/external.rs

@@ -102,7 +102,7 @@ mod it_evaluation {
 }
 
 mod stdin_evaluation {
-    use super::nu_error;
+    use super::{nu, nu_error};
     use nu_test_support::pipeline;
 
     #[test]
@@ -117,6 +117,21 @@ mod stdin_evaluation {
 
         assert_eq!(stderr, "");
     }
+
+    #[test]
+    fn does_not_block_indefinitely() {
+        let stdout = nu!(
+            cwd: ".",
+            pipeline(r#"
+                iecho yes
+                | chop
+                | chop
+                | first 1
+            "#
+        ));
+
+        assert_eq!(stdout, "y");
+    }
 }
 
 mod external_words {