Switch to Current-Thread Tokio Runtime

[ibraheemdev]

Summary

Move completely off tokio's multi-threaded runtime. We've slowly been making changes to be smarter about scheduling in various places instead of depending on tokio's general purpose work-stealing, notably #3627 and #4004. We now no longer benefit from the multi-threaded runtime, as we run on all I/O on the main thread. There's one remaining instance of block_in_place that can be swapped for rayon::spawn.

This change is a small performance improvement due to removing some unnecessary overhead of the multi-threaded runtime (e.g. spawning threads), but nothing major. It also removes some noise from profiles.

Test Plan

Benchmark 1: ./target/profiling/uv (resolve-warm)
  Time (mean ± σ):      14.9 ms ±   0.3 ms    [User: 3.0 ms, System: 17.3 ms]
  Range (min … max):    14.1 ms …  15.8 ms    169 runs
 
Benchmark 2: ./target/profiling/baseline (resolve-warm)
  Time (mean ± σ):      16.1 ms ±   0.3 ms    [User: 3.9 ms, System: 18.7 ms]
  Range (min … max):    15.1 ms …  17.3 ms    162 runs
 
Summary
  ./target/profiling/uv (resolve-warm) ran
    1.08 ± 0.03 times faster than ./target/profiling/baseline (resolve-warm)

[charliermarsh]

Trusting you on this one :)