Add way to listen for a block's change in reattach/promote state

[luca-moser]

We want to expose streams/events for clients to be notified for when a block needs to be re-attached or promoted. We previously added a partial solution to this through the TipScoreUpdates event in INX and MQTT but that will not catch blocks which fall out of the tip pool (while still not in need to being promoted/reattached) and then fall below max depth afterwards.

During confirmation of a milestone, we updated the root cone indices, which is the point where we can emit the events for the blocks which fall below max depth.