The pubsub library updates to clean up potential runtime errors. Every
so often we can end up with a nil message. Which either means we
dropped a message or we're attempting to process a message that isn't
there. This can lead to issues in people attempting to use the library.

Removing the complexity of knowing when to request the next message. It
removes the ambiguity of knowing if you have a next message or not. The
trick of knowing when next is blocking or not is also gone, as we can
use the existing data channel. If that's already full (it's a buffered
channel of 1), then just offer a default case in the select to allow the
bypassing of the blocking operation.

Message pointers in the implementation is gone, you either have a
message or you don't. We no longer need to check for empty. And in
doing so, the implementation iterates all the pending handlers at once,
without checking for data or next each iteration.

The interface is exactly the same, just the underlying queue
implementation.