threadpool: drop incomplete tasks on shutdown (#722)

## Motivation

When the thread pool shuts down, futures that have been polled at least once but not completed yet are simply leaked. We should drop them instead.

## Solution

Multiple changes are introduced:

* Tasks are assigned a home worker the first time they are polled.

* Each worker contains a set of tasks (`Arc<Task>`) it is home to. When a task is assigned a home worker, it is registered in that worker's set of tasks. When the task is completed, it is unregistered from the set.

* When the thread pool shuts down and after all worker threads stop, the remaining tasks in workers' sets are aborted, i.e. they are switched to the `Aborted` state and their `Future`s are dropped.

* The thread pool shutdown process is refactored to make it more robust. We don't  track the number of active threads manually anymore. Instead, there's  `Arc<ShutdownTrigger>` that aborts remaining tasks and completes the `Shutdown` future once it gets destroyed (when all `Worker`s and `ThreadPool` get dropped because they're the only ones to contain strong references to the `ShutdownTrigger`).

Closes #424 
Closes #428

tokio-threadpool/src/shutdown.rs

@@ -86,6 +86,11 @@ impl Drop for ShutdownTrigger {
         // Drain the global task queue.
         while self.queue.pop().is_some() {}
 
+        // Drop the remaining incomplete tasks and parkers assosicated with workers.
+        for worker in self.workers.iter() {
+            worker.shutdown();
+        }
+
         // Notify the task interested in shutdown.
         let mut inner = self.inner.lock().unwrap();
         inner.completed = true;