feat: update to Tokio 0.3

[hawkw]

This branch updates Hyper's Tokio dependency to v0.3.x. This continues
the work started by @Urhengulas in PR #2309, updating additional parts
of the codebase and fixing some bugs introduced in that branch.

There are a few important notes:

• h2 is currently a Git dependency, so this will block releasing a new
  Hyper version until a new h2 is published.

• Tokio 0.3 doesn't currently have vectored IO APIs, so Hyper has
  (hopefully temporarily) lost its vectored IO support

• AsyncRead::poll_read_buf and AsyncWrite::poll_write_buf are being
  moved to tokio-util. Currently, they're not available, so we fall
  back to slicing and advancing Bytes and BytesMut buffers
  ourselves. When support lands upstream, we should change back.

• Ideally, Tokio's new TcpSocket type should gain support for more of
  the socket options hyper needs to set. If this happens, we should be
  able to remove some messy code involving AsRawFd/FromRawFd and
  socket2.

• This also bumps Hyper's minimum supported Rust version to 1.45.2,
  which is Tokio's minimum supported version. Tokio 0.3 doesn't build
  on 1.39.0

Closes #2309

Signed-off-by: Eliza Weisman [email protected]
Co-authored-by: Urhengulas [email protected]

[seanmonstar]

Some of the benchmarks are still unhappy, they're guarded by the nightly cargo feature.

[seanmonstar]

Finished in #2319