Undertow listener's 'enabled' attribute should not require reload to go from false to true

In general an 'enabled' type attribute shouldn't require a reload to go from 'false' to 'true'. Turning something on generally isn't disruptive to other currently running services. It's turning things off that can be problematic.

While not a focus of this JIRA, possibly even the true -> false case could be supported via the "allow-resource-service-restart" => true header, although that needs a bit more thought. Over the years I've gone back and forth on the wisdom of making support for that more widespread. It adds flexibility at the risk of making it easier for users to screw up. These days though I'm trending back toward thinking it's a good thing to support.

Side note: 'enabled' attributes are better avoided in the first place.