update the proxy to use Tokio 0.3

This branch updates the proxy to use Tokio 0.3 and the Tokio 0.3
versions of various ecosystem crates. This includes `tower` 0.4 and
`bytes` 0.6, as well as the Tokio 0.3 versions of `tokio-util`, `hyper`,
`tonic`, etc. Due to API changes in Tokio and in other dependencies, it
was necessary to make some code changes as well as updating
dependencies, but there should be no functional change.

In particular:
* Tokio's support for vectored IO changed significantly in 0.3, so this
  branch updates our use of `AsyncWrite` to participate in the new
  vectored write APIs
* Hyper's HTTP/1.1 upgrade API changed in 0.14, so this branch changes the
  proxy's code for handling CONNECT to use the new API
* Tokio removed support for some socket options, which now need to be
  set using `socket2`
* Tokio removed the `poll_ready` method was removed from the bounded
  MPSC channel, so the proxy's buffers (`linkerd2-buffer` and the
  `buffer` module in `linkerd2-proxy-discover`) had to be switched to
  our own implementation (this merged separately, in PR #759).

Several ecosystem crates have yet to be released, so we depend on them
via Git dependencies for now. The patches in Cargo.toml can be
removed as other dependencies publish their Tokio 0.3 versions.