Add peek to AsyncQueue

[bhartnett]

In some of my code I found the need to do something like this:

# wait for the first block data to be put on the queue
# so that we can access the first block once available
while blockDataQueue.empty():
  await sleepAsync(100.milliseconds)
# peek but don't remove it so that it can be processed later
let firstBlock = blockDataQueue[0]

Having an async peek proc that waits for the first item to be added to the queue would be good. The std/deques supports peekFirst and peekLast as well so this PR adds these to the AsyncQueue.

[arnetheduck]

Suggested change

	proc peak*[T](aq: AsyncQueue[T]): Future[T] {.
	proc peek*[T](aq: AsyncQueue[T]): Future[T] {.

[bhartnett]

My bad, will fix the spelling shortly.

[arnetheduck]

there needs to be a test here that ensures that the following also works:

let a = q.peekFirst()
let b = q.popFirst()

both in that order and in the reverse order - where peek-after-pop should probably not trigger

[bhartnett]

I've added more tests as suggested.