Use the pre-succeeded void future when possible for async pipeline operations

[glbrntt]

Motivation:

We added synchronous pipeline operations to allow the caller to save
allocations when they know they are already on the correct event loop.
However, we missed a trick! In some cases the caller cannot guarantee
they are on the correct event loop and must use an asynchronous method
instead. If that method returns a void future and is called on the event
loop, then we can perform the operation synchronously and return a
cached void future.

Modifications:

• Add API to EventLoop for creating a 'completed' future with a
  Result (similar to EventLoopPromise.completeWith)
• Add an equivalent for making completed void futures
• Use these when asynchronously adding handlers and the caller is
  already on the right event loop.

Result:

• Fewer allocations on the happiest of happy paths when adding handlers
  asynchronously to a pipeline.

[Lukasa]

I'm happy with this, I'd like to let @weissi review the API additions as well.

[weissi]

This is very cool! We just have some extra parameters that (I think) we don't need :)

[weissi]

So technically, we don't need these two methods. We can just use the fully generic one and do:

if Success.self == Void.self {
    // code for makeCompletedVoidFuture here
} else {
   // code for makeCompletedFuture here
}

and because of the @inlinable we get the same perf and will have a nicer API. WDYT @Lukasa / @glbrntt ?

[Lukasa]

SGTM

[glbrntt]

Don't even need to do that I think, makeCompletedFuture does it for us :)

[weissi]

yay! thank you George!