Refactor lustate handling to be much more efficient

[neilcook]

Previously luastates were stored in an array. Worker threads accessed luastates by simply retrieving the "next" state in the array, regardless of whether that state was in use or not. This commit instead uses a pool approach, where worker threads pop the state off the aray while the state is in use, then return the state once it's no longer needed (automatically via destructor). A read-only copy of the entire array is maintained for the console/ control thread which needs to be able to call all luastates.

[github-actions]

Test Results

  2 files  ±0    2 suites  ±0   32m 9s ⏱️ ±0s
 70 tests ±0   70 ✅ ±0  0 💤 ±0  0 ❌ ±0 
140 runs  ±0  140 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit b2b1146. ± Comparison against base commit acbdfd9.

[chbruyand]

lgtm!