Threads.resize_nthreads! can take a significant time to infer

[KristofferC]

Looking at some package load time I saw quite a bit of time is spent in inferring Threads.resize_nthreads!.

For example, loading HTTP.jl takes:

julia> @time using HTTP
  0.323988 seconds (1.04 M allocations: 63.554 MiB, 2.47% gc time, 1.98% compilation time)

on 1.6 for me. Remove the resize_nthreads! calls in HTTP.jl (and URIs.jl) makes it take:

 julia> @time using HTTP
  0.085508 seconds (66.35 k allocations: 6.057 MiB, 2.88% compilation time)

Maybe the deepcopy in the function makes it expensive to infer?