## 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#424Closes#428
`inner` is a fitting name for variables of type named `Inner`, but in other cases I find them confusing - sometimes `inner` refers to a `Pool`, sometimes to a `Sender`. I renamed a bunch of variables named `inner` to be more descriptive.
This PR is the first step in an effort of splitting https://github.com/tokio-rs/tokio/pull/722#issuecomment-439552671 into multiple PRs.
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## Motivation
Now that each worker thread drives its own reactor, reactors have to be driven until the threadpool shuts down. We mustn't use the `keep_alive` setting to shut down a worker thread if it doesn't receive an event from the reactor for a certain duration of time.
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## Solution
Just ignore the `keep_alive` setting when parking in `Worker::sleep`.
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@jonhoo reported a panic in the call to `LocalKey::with`, which occurs
when the reactor is dropped in the middle of TLS teardown. This PR
changes the call to `LocalKey::try_with` and handles the case when the
thread-local value has already been destroyed.
