Minor refactor on Thread pool

[frodsan]

• Puma::Threadpool uses only one constant from the constants file. At the beginning, I was thinking to add the missing require to const.rb to the ThreadPool, but I think it's better to just move the constant to where it's used.
• I removed the check for timeout == 0 since the times block will not be executed if it's zero.

[evanphx]

I believe this is wrong because it remove the call to Thread#raise when 0 is used.

[frodsan]

This is the code now:

when 0
  threads.each do |t|
    t.raise ForceShutdown
  end

  threads.each do |t|
    t.join SHUTDOWN_GRACE_TIME
  end
else
  timeout.times do
    # ...
  end

  threads.each do |t|
    t.raise ForceShutdown
  end

  threads.each do |t|
    t.join SHUTDOWN_GRACE_TIME
  end
end

And this is the proposed change:

else
  # If timeout is `0`, it will skip this part,
  # and raise (force shutdown) immediately
  timeout.times do
    # ...
  end

  threads.each do |t|
    t.raise ForceShutdown
  end

  threads.each do |t|
    t.join SHUTDOWN_GRACE_TIME
  end
end

I think in both scenarios #raise is called, when timeout is equal or greater than 0.

[nateberkopec]

Makes sense to me - clever use of 0.times.