ThrottleExecutor can leak thread when not shutdown()

[rohanpm]

Despite the issue #87 fix, which fixed most basic cases of thread leaks, ThrottleExecutor can still leak threads if it's not shut down.

It can be reproduced by adding a new test like this:

def test_no_leak_on_completed_futures(executor_ctor):
    no_extra_threads = partial(assert_no_extra_threads, thread_names())

    executor = executor_ctor()
    futures = [executor.submit(mult2, n) for n in [10, 20, 30]]
    results = [f.result() for f in futures]
    del executor
    del futures

    assert_soon(no_extra_threads)

...and running it with an Executors.with_poll(...).with_throttle(2).

The problem is that _submit_loop can still hold a reference to futures (and hence executor) via to_submit and job locals.