executor: add executor::exit (#1155)

This allows blocking on executors from within a `threadpool::blocking` call.

tokio-threadpool/src/blocking.rs

@@ -1,6 +1,7 @@
 use worker::Worker;
 
 use futures::Poll;
+use tokio_executor;
 
 use std::error::Error;
 use std::fmt;
@@ -142,8 +143,11 @@ where
     // If the transition cannot happen, exit early
     try_ready!(res);
 
-    // Currently in blocking mode, so call the inner closure
-    let ret = f();
+    // Currently in blocking mode, so call the inner closure.
+    //
+    // "Exit" the current executor in case the blocking function wants
+    // to call a different executor.
+    let ret = tokio_executor::exit(move || f());
 
     // Try to transition out of blocking mode. This is a fast path that takes
     // back ownership of the worker if the worker handoff didn't complete yet.