Update to Tokio 0.3

[SriRamanujam]

No description provided.

[djc]

The AsyncFd PR is here: tokio-rs/tokio#2903. I think we need to implement our UDP wrapper. It also looks like the 0.7 mio UdpSocket has lost its only_v6() method, so we have to get that reimplemented upstream or figure out some other way to detect dual-stack sockets.

[djc]

(I have been discussing with an Apple engineer from the quicdev Slack whether there's a better way to do things on Darwin or whether there's a bug on their side, I'll see what I can get out of that.)

[djc]

Very WIP PR in #873.

[Ralith]

I'm concerned that the move to AsyncFd doesn't give us a way forward for specialized UDP operations on non-Unix platforms.

[djc]

They're open to feedback on the PR, I think.

[Darksonn]

We are very open to feedback on the best way forward. The reason it didn't make it for 0.3.0 was unanswered questions on the best way to design it.

[Ralith]

Discussion on the Tokio discord revealed that we don't actually have the capacity for specialized UDP operations outside Unix on 0.2 either, so we're good to proceed once AsyncFd or equivalent is in.

[Ralith]

See also tokio-rs/tokio#2968.

[Darksonn]

We have released AsyncFd.

[djc]

Let's use #856 for this.