Don't rely on tokio_unstable config flag

[mzabaluev]

Description

Tokio does not guarantee semver stability of unstable features, so it's a bad idea to use this flag. It's also annoying to have to override rustflags everywhere.
Remove the tokio_unstable flag from local cargo config and change the affected code to not use unstable features.

Type of Change

• Refactoring

Which Components or Systems Does This Change Impact?

• Aptos Framework
• Aptos CLI/SDK
• Developer Infrastructure

How Has This Been Tested?

cargo check -p movement should build, while .cargo/config.toml has been modified to not pass --cfg tokio_unstable to rustc.

Key Areas to Review

Whether the changes in runtime behavior could be harmful. I don't think so, it's a perf tweak and before we have proof this degrades performance, I think all such tweaks are fair game to remove.

Checklist

• I have read and followed the CONTRIBUTING doc
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I identified and added all stakeholders and component owners affected by this change as reviewers
• I tested both happy and unhappy path of the functionality
• I have made corresponding changes to the documentation