[Proposal] Add a CountDownLatch implementation

[navaronbracke]

I recently stumbled upon a use case where I could use a count down latch implementation.

However, I didn't find any such implementation in package:async.
I propose we add one here.

In terms of API surface, all it needs to do in my opinion is the following:

• keep track of a "count" as a internal counter and as an internal stream of said counter
• provide a method to increment / decrement the latch
• provide a Future<void> wait() method, which is essentially an await firstWhere((count) => count == 0);
• provide a void reset(); which just resets the latch