Add new tracer event that reports a block's propagation delay within our node

@karknu and @njd42 pointed out that a a helpful diagnostic metric on running nodes would be (statistics about) the delay between receiving a block and when we began offering it to peers. In some sense, this is our node's internal contribution to the that block's propagation delay (at least for p2p connections whose shortest path involves us).

This Issue is to create a tracer event that includes that specific delay along with the block's metadata and likely additional information about the corresponding chain selection event (eg if it was a roll back, eg sizes of other blocks).

There are some open questions, such as: Would it be ok to only emit this event for some block when that block becomes our _tip_? Recall that a block can be on our chain without having ever been our tip (eg if it's in the prefix of short fork that later wins out). Only emitting the event for our tip would be quite a bit easier to implement, eg (boils down to adding an ingress timestamp to the add-block queue, I think).