io: use intrusive wait list for I/O driver (#2828)

This refactors I/O registration in a few ways:

- Cleans up the cached readiness in `PollEvented`. This cache used to
  be helpful when readiness was a linked list of `*mut Node`s in
  `Registration`. Previous refactors have turned `Registration` into just
  an `AtomicUsize` holding the current readiness, so the cache is just
  extra work and complexity. Gone.
- Polling the `Registration` for readiness now gives a `ReadyEvent`,
  which includes the driver tick. This event must be passed back into
  `clear_readiness`, so that the readiness is only cleared from `Registration`
  if the tick hasn't changed. Previously, it was possible to clear the
  readiness even though another thread had *just* polled the driver and
  found the socket ready again.
- Registration now also contains an `async fn readiness`, which stores
  wakers in an instrusive linked list. This allows an unbounded number
  of tasks to register for readiness (previously, only 1 per direction (read
  and write)). By using the intrusive linked list, there is no concern of
  leaking the storage of the wakers, since they are stored inside the `async fn`
  and released when the future is dropped.
- Registration retains a `poll_readiness(Direction)` method, to support
  `AsyncRead` and `AsyncWrite`. They aren't able to use `async fn`s, and
  so there are 2 reserved slots for those methods.
- IO types where it makes sense to have multiple tasks waiting on them
  now take advantage of this new `async fn readiness`, such as `UdpSocket`
  and `UnixDatagram`.

Additionally, this makes the `io-driver` "feature" internal-only (no longer
documented, not part of public API), and adds a second internal-only
feature, `io-readiness`, to group together linked list part of registration
that is only used by some of the IO types.

After a bit of discussion, changing stream-based transports (like
`TcpStream`) to have `async fn read(&self)` is punted, since that
is likely too easy of a footgun to activate.

Refs: #2779, #2728

tokio-util/src/cfg.rs

@@ -18,6 +18,7 @@ macro_rules! cfg_compat {
     }
 }
 
+/*
 macro_rules! cfg_udp {
     ($($item:item)*) => {
         $(
@@ -27,6 +28,7 @@ macro_rules! cfg_udp {
         )*
     }
 }
+*/
 
 macro_rules! cfg_io {
     ($($item:item)*) => {

tokio-util/src/lib.rs

@@ -30,9 +30,14 @@ cfg_codec! {
     pub mod codec;
 }
 
+/*
+Disabled due to removal of poll_ functions on UdpSocket.
+
+See https://github.com/tokio-rs/tokio/issues/2830
 cfg_udp! {
     pub mod udp;
 }
+*/
 
 cfg_compat! {
     pub mod compat;