upgrade: Tokio 0.2

[bartlomieju]

This PR upgrades Tokio version in CLI.

There are currently two pending issues:

• new tokio::fs::File doesn't expose .into_std() method, which means file cannot be passed to subprocess as stdio (fixed in Tokio 0.2.2)
• awaiting reqwest update, ie. fetch API doesn't work yet

Ref #3387, tokio-rs/tokio#1852

[bartlomieju]

FYI: I hit some problems upgrading reqwest due to API changes there (Decoder which we use to implement HttpBody resource is no longer available). Anyway, still waiting on reqwest release, I'll coordinate with them to make this work.

[bartlomieju]

reqwest is close to 0.10.0 release

[ry]

Thank you very much for working on this! Please add TODOs and/or an issue describing the flush situation. It's not ideal - but I think we all agree that this needs to land. Otherwise LGTM.

[95th]

This PR has reduced the no. of syscalls

There is also increase of one thread (Perhaps this is temporary due to uses of block_on at various places).

[hayd]

Did this add native-tls as a requirement (in hyper?). It seems like openssl is now required (at least to build) on Amazon Linux, where previously we were using rustls?
https://github.com/hayd/deno-docker/runs/371640720

In the docs: https://hyper.rs/guides/client/configuration/

This means that you could pick a different TLS implementation than the one chosen by hyper-tls, such as rustls.

But I'm not really sure how!