Support cancellation of concurrent tasks
Currently a cancellation event sent to a worker with `concurrency` > 1
will always cancel the most recent task.
This commit refactors the `_ChildWorker` class to exclusively send the
`Cancel` event when receiving a `CancelRequest` and then handle the
cancellation in the event loop.
For the async `_aloop` we now use a weak map to track all in flight
prediction tasks. On cancel, we retrieve the task from the map and
cancel it.
For the standard `_loop` method we send the cancellation signal to the
process as before. It's not entirely clear why the child needs to send
the signal to itself vs raising the `CancellationException` directly
but this commit leaves it as-is for simplicity.
