HTTP Server should allow custom concurrency models

[stakach]

Splitting out this function makes it simple to implement a custom HTTP::Server using a Fiber pool i.e.

require "fiberpool"

class MyServer < HTTP::Server
  @my_pool : FiberPool = FiberPool.new(200)

  protected def dispatch(io)
    @my_pool.run { handle_client(io) }
  end
end

for example

[HertzDevil]

I am not entirely sure if it is a good idea to expect people to inherit from HTTP::Server. This is not a public API, so it has no stability guarantees and can be easily broken by things like #13080, yet this is not something meant to be publicly callable from the outside either. I am approving this as a refactor, but please note that this can break in a minor release without prior notice given the current state of affairs.

If we decide that protected or private methods can be part of a public API because of inheritance, we should find a way to show them in the docs in the future (e.g. something with the opposite effect of :nodoc:). Shard authors might want that too.