Change need_read and need_write to return an error

This commit is targeted at solving tokio-rs/tokio-core#12 and incorporates the
solution from tokio-rs/tokio-core#17. Namely the `need_read` and `need_write`
functions on `PollEvented` now return an error when the connected reactor has
gone away and the task cannot be blocked. This will typically naturally
translate to errors being returned by various connected I/O objects and should
help tear down the world in a clean-ish fashion.

tests/drop-core.rs

@@ -0,0 +1,39 @@
+extern crate tokio;
+extern crate futures;
+
+use std::thread;
+
+use futures::future;
+use futures::prelude::*;
+use futures::sync::oneshot;
+use tokio::net::TcpListener;
+use tokio::reactor::Core;
+
+#[test]
+fn tcp_doesnt_block() {
+    let core = Core::new().unwrap();
+    let handle = core.handle();
+    let listener = TcpListener::bind(&"127.0.0.1:0".parse().unwrap(), &handle).unwrap();
+    drop(core);
+    assert!(listener.incoming().wait().next().unwrap().is_err());
+}
+
+#[test]
+fn drop_wakes() {
+    let core = Core::new().unwrap();
+    let handle = core.handle();
+    let listener = TcpListener::bind(&"127.0.0.1:0".parse().unwrap(), &handle).unwrap();
+    let (tx, rx) = oneshot::channel::<()>();
+    let t = thread::spawn(move || {
+        let incoming = listener.incoming();
+        let new_socket = incoming.into_future().map_err(|_| ());
+        let drop_tx = future::lazy(|| {
+            drop(tx);
+            future::ok(())
+        });
+        assert!(new_socket.join(drop_tx).wait().is_err());
+    });
+    drop(rx.wait());
+    drop(core);
+    t.join().unwrap();
+}