io: clean up I/O driver shutdown process

[carllerche]

The I/O driver needs some restructuring in order to clean up the shutdown process.

Currently, internals are structured such that there is a cycle between ScheduledIo (internal I/O resource state) and io::driver::Inner. The driver's internal struct holds a slab containing all the resource's ScheduledIo instances. Each ScheduleIo tracks wakers that should be notified when I/O events arrive. The wakers hold a reference to the driver's inner structure in order to unpark an I/O driver blocked in poll.

                        +---------------+
       +----------------+    Wakers     |
       |                +-------+-------+
       |                        ^
       v                        |
+------+------+         +-------+-------+
| Inner/Slab  +-------->|  ScheduledIo  |
+------+------+         +-------+-------+
       ^                        ^
       |                        |
+------+------+         +-------+------+
|    Driver   |         | Registration |
+-------------+         +--------------+

#3477 fixes the memory leak due to the cycle by ensuring wakers are dropped when a Registration is dropped. This breaks the cycle. However, Wakers may hold arbitrary structures. It is possible to implement a Waker that also holds a Registration. In this case, the cycle would not be broken leading again to a memory leak. This is a fairly convoluted scenario and probably does not happen much in practice, but it should still be fixed.

To fix this, we need the I/O driver shutdown process to clear any wakers being held by ScheduledIo instances. To do this without introducing a race condition is a bit tricky. First, a "is_shut_down" flag needs to be set atomically in a way that ensures no further ScheduledIo instances may be created. Then, each ScheduledIo must be iterated and a shutdown flag must be set on it. This can be set as part of the "readiness" field. If a waker is stored in the ScheduledIo it should be notified & dropped.

[zaharidichev]

@carllerche Can I take this one? Should #3564, merge first?

[carllerche]

I think this is fairly standalone. It mostly requires adding the ability to shutdown the internal slab itself. Feel free to take a stab at it and ask Qs on discord 👍