Implement PublishAsync

[mrpmorris]

Proposed change

Publish looks as though it blocks the current thread, is this the case? (If not, I assume it doesn't guarantee the message is received before the thread continues?)

Would it be possible to have an awaitable implementation to improve server performance?

Use case

Concurrency improvement.

Contribution

Yes.

[scottf]

@mrpmorris I'm closing this. You are welcome to submit a PR, although this functionality can already be accomplished by the developer wrapping the current synchronous calls themselves.