Improve engine tracking of configured VIP addresses.

[baptr]

Otherwise the engine can leave configured (and try to act upon) no longer used VIP addresses.

• When receiving a vserver config update, check that the previously configured unicast VIP addresses are still needed, and remove them if not.
• Better document relevant vserver fields.
• Ensure vips and lbVservers maps are in-sync with the interface by pruning as necessary.
• Implement a stub lb interface for testing that allows introspection of live configuration.
• Use it to validate state during a re-ip transition.
• Other minor test style cleanup.