Auto reload config on changes (requires EventThread)

[bradh352]

Automatically detect configuration changes and reload. On systems which provide notification mechanisms, use those, otherwise fallback to polling. When a system configuration change is detected, it asynchronously applies the configuration in order to ensure it is a non-blocking operation for any queries which may still be being processed.

On Windows, however, changes aren't detected if a user manually sets/changes the DNS servers on an interface, it doesn't appear there is any mechanism capable of this. We are relying on NotifyIpInterfaceChange() for notifications.

Fixes Issue: #613

[bradh352]

@bagder any comments/questions/concerns with this? I'm just enabling it always if EventThread is enabled.