Update `tokio` to 1.0

[stanislav-tkach]

Many crates (including rusoto) have already been migrated to the 1.0 version. It would be nice to update the runtime, too.

[seanpianka]

There's an active fork of this project, maintained here, which has already merged updates to Tokio 1.0, along with numerous other improvements contributed by the community. These same contributions have been blocked for months as PRs-awaiting-review in this project, so I recommend to migrate there.

Here's how I seamlessly replaced this version with the new fork in my Cargo.toml:

lambda_http = { package = "netlify_lambda_http", version = "0.2.0" }