fix(#198): limit tokio threads to 4 (+ main)

[grantlemons]

Tokio has two different types of threads:

• Worker threads: for async things
• Blocking threads: main and spawn_blocking

I limited the number of worker threads to 4, but the actual number of threads should hover around 5.
I'm not certain whether we spawn any blocking threads other than main, but if we do it should be minimal.

[elijah-potter]

Looks good to me. Unless tower-lsp spawns any blocking threads, there isn't anywhere where we do.