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/worker/mod.rs

@@ -309,7 +309,7 @@ impl Worker {
         found_work
     }
 
-    fn run_task(&self, task: Task, notify: &Arc<Notifier>, sender: &mut Sender) {
+    fn run_task(&self, task: Arc<Task>, notify: &Arc<Notifier>, sender: &mut Sender) {
         use task::Run::*;
 
         match task.run(notify, sender) {