tokio-1.16.0

Fixes a soundness bug in `io::Take` ([#4428]). The unsoundness is exposed when
leaking memory in the given `AsyncRead` implementation and then overwriting the
supplied buffer:

```rust
impl AsyncRead for Buggy {
    fn poll_read(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut ReadBuf<'_>
    ) -> Poll<Result<()>> {
      let new_buf = vec![0; 5].leak();
      *buf = ReadBuf::new(new_buf);
      buf.put_slice(b"hello");
      Poll::Ready(Ok(()))
    }
}
```

Also, this release includes improvements to the multi-threaded scheduler that
can increase throughput by up to 20% in some cases ([#4383]).

### Fixed

- io: **soundness** don't expose uninitialized memory when using `io::Take` in edge case ([#4428])
- fs: ensure `File::write` results in a `write` syscall when the runtime shuts down ([#4316])
- process: drop pipe after child exits in `wait_with_output` ([#4315])
- rt: improve error message when spawning a thread fails ([#4398])
- rt: reduce false-positive thread wakups in the multi-threaded scheduler ([#4383])
- sync: don't inherit `Send` from `parking_lot::*Guard` ([#4359])

### Added

- net: `TcpSocket::linger()` and `set_linger()` ([#4324])
- net: impl `UnwindSafe` for socket types ([#4384])
- rt: impl `UnwindSafe` for `JoinHandle` ([#4418])
- sync: `watch::Receiver::has_changed()` ([#4342])
- sync: `oneshot::Receiver::blocking_recv()` ([#4334])
- sync: `RwLock` blocking operations ([#4425])

### Unstable

The following changes only apply when building with `--cfg tokio_unstable`

- rt: **breaking change** overhaul runtime metrics API ([#4373])

[#4428]: https://github.com/tokio-rs/tokio/pull/4428
[#4316]: https://github.com/tokio-rs/tokio/pull/4316
[#4315]: https://github.com/tokio-rs/tokio/pull/4315
[#4398]: https://github.com/tokio-rs/tokio/pull/4398
[#4383]: https://github.com/tokio-rs/tokio/pull/4383
[#4359]: https://github.com/tokio-rs/tokio/pull/4359
[#4324]: https://github.com/tokio-rs/tokio/pull/4324
[#4384]: https://github.com/tokio-rs/tokio/pull/4384
[#4418]: https://github.com/tokio-rs/tokio/pull/4418
[#4342]: https://github.com/tokio-rs/tokio/pull/4342
[#4334]: https://github.com/tokio-rs/tokio/pull/4334
[#4425]: https://github.com/tokio-rs/tokio/pull/4425
[#4373]: https://github.com/tokio-rs/tokio/pull/4373