Prefer tokio runtime everywhere

[thomaseizinger]

Description

Historically, examples and tests in rust-libp2p have used a mix of the async-std and tokio runtime. These days, both runtimes are equally well supported by the library but the examples and tests are still mixed. For now, we want to keep supporting async-std but all examples and tests should use the tokio runtime.

Motivation

The tokio runtime is much more well established and better maintained. Users should be able to use other runtimes or bring their own.

Current Implementation

Both runtimes are being mixed.