When a new IP is detected, remove that IP on other hosts if it exists

In reactive L2 forwarding we want to make sure ONOS only has each IP address attached to one host at a time, so we know where to direct packets for that IP.

When the provider notifies the core that the IP information for a host has changed, we should check in the store if any other host has the IP and remove it from the older host entry.

This should be configurable (default = enabled).