Upgrade to Tokio v0.3

[seanmonstar]

Tokio v0.3 just released, we should upgrade as part of hyper v0.14.

Prerequisites:

• Upgrade h2 to Tokio v0.3
• Add back functionality to Tokio v0.3
  • Reintroduce oneshot::Sender::poll_closed Make oneshot::Sender::poll_closed public again tokio-rs/tokio#3032
  • Add back TCP socket options (set_keepalive, set_send_buffer_size, set_recv_buffer_size)
  • Add back vectored write support

[alex]

hyperium/h2#491 is a PR for updating h2.