Make threadpool optional

[petoknm]

I am using touche in single threaded mode and I was expecting that my config (below) would eliminate the code from threadpool crate if unused, but it was not able to completely remove it. I assume this was because of some use of it in Server type.

[profile.release]
opt-level = "z"
lto = true
codegen-units = 1

With this MR, I was able to fully eliminate threadpool crate's code from my final binary.

[reu]

That's some excellent work! Just a requesting a minor change but besides that this is ready to go! Thanks!

[petoknm]

The CI jobs scripts might need a tweak, currently they use --no-default-features --features server.

[reu]

Thanks a lot @petoknm, looks great!