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/lib.rs

@@ -79,6 +79,7 @@
 
 extern crate tokio_executor;
 
+extern crate crossbeam;
 extern crate crossbeam_channel;
 extern crate crossbeam_deque as deque;
 extern crate crossbeam_utils;
@@ -86,6 +87,7 @@ extern crate crossbeam_utils;
 extern crate futures;
 extern crate num_cpus;
 extern crate rand;
+extern crate slab;
 
 #[macro_use]
 extern crate log;