Use delete to avoid memory leak

[jayantk]

Hey there! I'm using your library via a somewhat involved chain of dependencies, and I've been running into a memory leak. I debugged it using the node's heap inspector and I found that the queue object here continuously grows in size because its keys never get deleted. The object looks like {1: null, 2: null, 3: null, ...}.

The fix is pretty easy -- just use delete to remove the key from the object in addition to the value.

The main change is the delete thing ^. Some other files seem to have changed when I made the change above and ran test and I'm not sure why. I'm kind of a noob to the js ecosystem, so please tell me if I should revert some of those changes.

[mkozjak]

@jayantk you did everything right! Thanks so much!