refactor(relay): keep connection alive while we are using it

[thomaseizinger]

Description

Previously, the relay client was applying a 10 second timeout to idle connections. Instead, we now compute the connection keep alive based on whether we are still using the connection.

This makes all tests pass apart from 1: reuse_connection. This makes sense as that connection is immediately idle once dialed. To make that test pass, we configure a global idle connection timeout of 1 second.

In a real-world scenario, reusing a connection only applies if the connection is still alive due to other protocols being active.

Related: #3844.

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[mxinden]

Makes sense to me. Thanks.