Run shutdown handlers is paralle

[gajus]

Currently the shutdown handlers are executed sequentially.

Running shutdown handlers in parallel would potentially shorten the shutdown time.

Refer to https://github.com/gajus/lightship/issues/5#issuecomment-451775408 discussion.

[memelet]

I agree with @dcherman, that dependencies should be handled by registered handler.

[gajus]

This introduces unnecessary ambiguity. There is nothing stopping user from simply implementing their own collection of methods that need to be run in parallel, i.e.

registerShutdownHandler(async () => {
  await Promise.all([
    userCollectionOfShutdownHandlers,
  ]);
});