[comm-lib] Use futures_util::Ready to simplify code

Summary:
Minor change that greatly simplifies future code in the stacks.

The std::future::Ready::into_inner() is unstable so we had to use async { } block and boxed future.
Used futures_util::Ready instead, which has this method stable.

Test Plan: Cargo build

Reviewers: varun, will, kamil

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12411