Upgrade the AVM to the latest version of tokio

[cdmistman]

Fixes #369

[CLAassistant]

All committers have signed the CLA.

[cdmistman]

forgot about actually upgrading to tokio 1, next commit should finish this PR

[cdmistman]

MacOS CI (and local build) fails because mio:0.7.7 is using libc:0.2.69, whereas libc:0.2.71 is the minimum required version to use libc::TCP_KEEP_INTVL and libc::TCP_CNT (introduced in this PR). See tokio-rs/mio#1433 for more info. Blocked on tokio-rs/mio#1434 merge, mio:0.7.8, and all mio-based dependencies converging on the updated crate.

[dfellis]

and all mio-based dependencies converging on the updated crate.

Is it not possible to override the dependencies of other dependencies in Rust? Worst-case by hand-editing Cargo.lock? I though that's what that file was for.