Improve Relay after Persisting blocks (relay last 2 instead of first 1)

[jsolman]

Had a discussion with @vncoelho about the code that happens on OnNewBlock and during the discussion I noticed something that looked sub-optimal related to how the code determines which blocks to relay upon persist.

The code was only relaying the first block persisted in the case that there are multiple blocks also being persisted from the block cache. Since the idea is to keep nodes on the chain aware of the the most recent blocks, it should be relaying the last block rather than the first one. The code change I've made here will relay the last 2 blocks when multiple were persisted at once by taking some from the block cache.

@vncoelho indicated he would be able to do some testing with this, and I'll give it some testing also and update notes here after.

I believe this change will help nodes to obtain the latest blocks more quickly.

[jsolman]

I did some testing on this already and it looks good so far.

[vncoelho]

Thanks, @jsolman! You are genius.
It is great to talk to you in person.
I already tested yesterday as well.
We merged it inside the new Consensus proposal, thus we will be often testing this during the next days.
Let's go for the next step 🗡️ : TaskManager request blocks