upgrade to tokio 1.0 ecosystem

[tdyas]

Upgrade to the Tokio 1.0 ecosystem.

[tdyas]

The roundtrip_cancellation test is currently failing with:

test tests::roundtrip_cancellation ... FAILED

failures:

---- tests::roundtrip_cancellation stdout ----
Got connection: PollEvented { io: Some(TcpStream { addr: 127.0.0.1:51041, peer: 127.0.0.1:51044, fd: 38 }) }
thread 'tests::roundtrip_cancellation' panicked at 'assertion failed: `(left == right)`
  left: `ExitCode(-2)`,
 right: `ExitCode(123)`', nails/src/lib.rs:159:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    tests::roundtrip_cancellation

[tdyas]

Interestingly, fixed the hang by using notify_one instead of notify_waiters in ConstantNail. My choice of notify_waiters as if it were a condition variable was not correct. notified apparently only works with notify_one.

[stuhood]

Thanks!