Add Async API for new Connections

[ottlukas]

Currently we only support an async API for Requests.
But as creating a new Connection is sometimes even slower it would be good to provide an additional asyc API which returns a Future<PlcConnection> and leaves it to the Client to handle asynchronity.

Imported from Jira PLC4X-136. Original Jira may contain additional context.
Reported by: julian.feinauer.

[chrisdutz]

Can we close this issue?

[ottlukas]

As this has been opened by @JulianFeinauer I assume yes. and close with this comment.