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/pool/mod.rs

@@ -45,12 +45,6 @@ pub(crate) struct Pool {
     // Stack tracking sleeping workers.
     sleep_stack: CachePadded<worker::Stack>,
 
-    // Number of workers that haven't reached the final state of shutdown
-    //
-    // This is only used to know when to single `shutdown_task` once the
-    // shutdown process has completed.
-    pub num_workers: AtomicUsize,
-
     // Worker state
     //
     // A worker is a thread that is processing the work queue and polling
@@ -122,7 +116,6 @@ impl Pool {
         let ret = Pool {
             state: CachePadded::new(AtomicUsize::new(State::new().into())),
             sleep_stack: CachePadded::new(worker::Stack::new()),
-            num_workers: AtomicUsize::new(0),
             workers,
             queue,
             trigger,
@@ -313,7 +306,6 @@ impl Pool {
         }
 
         let trigger = match self.trigger.upgrade() {
-            // The pool is shutting down.
             None => {
                 // The pool is shutting down.
                 return;