I/O resources lazily bind to reactor. (#160)

This patch makes a significant change to how I/O resources bind to a
reactor. Currently, an I/O resource (TCP, UDP, PollEvented) will bind
itself with a reactor upon creation.

First, some history.

Originally, tokio-core required that I/O resources be explicitly
associated with a reactor upon creation by passing in a `&Handle`. Tokio
reform introduced a default reactor. If I/O resources do not specify a
reactor upon creation, then the default reactor is used.

However, futures tend to favor being lazy. Creating a future should do
no work, instead it is defining a computation to be performed once the
future is executed. Binding an I/O resource with a reactor on creation
goes against this pattern.

This patch fixes this by allowing I/O resources to lazily bind to a
reactor. An explicit `&Handle` can still be used on creation, but if no
reactor is specified, then the default reactor is used. However, this
binding happens during execution time (read / write) and not creation.

src/reactor/background.rs

@@ -4,9 +4,10 @@ use std::sync::Arc;
 use std::sync::atomic::AtomicUsize;
 use std::sync::atomic::Ordering::SeqCst;
 
+use atomic_task::AtomicTask;
+
 use reactor::{Reactor, Handle};
 use futures::{Future, Async, Poll};
-use futures::task::AtomicTask;
 
 /// Handle to the reactor running on a background thread.
 #[derive(Debug)]
@@ -117,6 +118,8 @@ impl Drop for Background {
             None => return,
         };
 
+        inner.shutdown_now();
+
         let shutdown = Shutdown { inner };
         let _ = shutdown.wait();
     }