Use single-threaded runtime in tokio

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

Generally there is very low CPU and I/O utilization by Slumber, so there's no benefit in distributing work over multiple tasks. Uses a single thread should cut down on wasted context switches, and also allow for simplifying code by eliminating the need for the mpsc message queue.

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

If someone is trying to send a ton of requests at once, it's possible we start to become CPU-bound by the single thread. I think this is very unlikely though. We could always switch back to multi-threading if necessary.

QA

How did you test this?

Manually ran tests and reloaded collection in the TUI

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked