Refactor threadpool task types (#300)

Replaces homegrown Arc with std Arc

Is this safer? Unknown. At least we don't have to maintain an arc
implementation anymore. This will also make it easier to filter out tsan
false positives.

Also split task/mod.rs into multiple files.

tokio-threadpool/src/sender.rs

@@ -168,7 +168,7 @@ impl<'a> tokio_executor::Executor for &'a Sender {
         // execution.
 
         // Create a new task for the future
-        let task = Task::new(future);
+        let task = Arc::new(Task::new(future));
 
         self.inner.submit(task, &self.inner);