Replace map[a]bool with map[a]struct{}

The later uses no memory storage for values and provides the same
functionality.
Use chan struct{} for semaphores

With semaphores we don't actually care about the value passed in. It
makes sense to use a 0 bytes type in these cases.
There is also the added benefit of compiler optimisations for this
specific use case as described here:
https://docs.google.com/document/d/1yIAYmbvL3JxOKOjuCyon7JhW4cSv1wy5hC0ApeGMV9s/pub