swarm/src/connection: Prioritize handler over connection

[mxinden]

Description

Prioritize work in handler over work on connection, thus prioritizing local work
over work coming from a remote.

I would like to make prioritizing local work over work coming from a remote a pattern throughout the codebase. More to come.

I will run this change on a larger machine to make sure it does not introduce any subtle errors.

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.
  • I don't think this change needs a call-out in the changelog.

[thomaseizinger]

Concept ACK

Was there a specific motivation for this change in prioritization?

[mxinden]

Was there a specific motivation for this change in prioritization?

General theme of prioritizing local work over work from remote. I hope for this to improve both performance and security, though applying the pattern across the stack is likely required for that to show.

Thanks for the review @thomaseizinger!