[Scrubber] Keepalive mechanism to invalidate the registration

[JeffreyDevloo]

A small entry can be kept for every scrubber which every scrubber updates. After a certain lease period, another scrubber can cleanup the old scrubbers registrations and continue itself

[JeffreyDevloo]

We've had a few encounters where restarting all ovs-workers did not invalidate the queue.
It'd be best to re-iterate the invalidaton mechanic

[JeffreyDevloo]

Will be implementing a generic keepalive to be used for:

• Ensure single
• Scrubber registrations