Async module reference implementation

[zaucy]

An implementation of the async module functions to demonstrate a correct implementation. Useful for us at @seaube and other implementers.

depends on #78

• connect/disconnect methods for the reference implementation should have a correct and incorrect string to demonstrate successful and unsuccessful attempts. Simply accepting "good" or "bad" as a connection string would be sufficient, but I'll leave that up to the author
• after connect has occured the implementation could simply execute on another thread in the background or do something else entirely. Only requirement is that execution does NOT happen during a flush call. Locking is fine. Authors choice.
• execution options must merge if enqueueing happens on the same tick(s). See async api enqueue options documentation. (once #79 is merged)
• DONT REINVENT THE WHEEL - Use the core module to implement the async api. The reference implementation must not implement any of the module methods.