Retain Cycles

[Coledunsby]

There are several retain cycles in AdaWebHost:

• reachability.whenReachable closure
• DispatchQueue.main.asyncAfter closure
• userContentController.add(self, name: "...") (x3)

I started resolving these (#24) but realized that this would likely break the scenario of launching the view controller multiple times from the same AdaWebHost instance. I would prefer to use AdaWebHost as a non singleton instance but the retain cycles currently prevent that.