Problem: Computing number of threads for parallel asynchronous
computation using `uv.cpu_info()` can be slow. This is especially
noticeable since it is pre-computed on every `require('vim.pack')` and
not only when parallelism is needed.
Solution: Use `uv.available_parallelism()` to compute number of threads
in a helper function.
(cherry picked from commit 3cba8df041)
fdf94493cf
