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/Cargo.toml

@@ -20,11 +20,13 @@ categories = ["concurrency", "asynchronous"]
 [dependencies]
 tokio-executor = { version = "0.1.2", path = "../tokio-executor" }
 futures = "0.1.19"
+crossbeam = "0.6.0"
 crossbeam-channel = "0.3.3"
 crossbeam-deque = "0.6.1"
 crossbeam-utils = "0.6.2"
 num_cpus = "1.2"
 rand = "0.6"
+slab = "0.4.1"
 log = "0.4"
 
 [dev-dependencies]