Bug: fix spine smith process

[iltoga]

Description

update blocksmith selection to match the block spine public keys from previous block, both when generating new blocks and validating the ones coming from peers.

This is because, when downloading the spine blocks it could happen that the node is unable to validate new blocks if it they are smithed by newly registered nodes that have their public keys included in the same blocks the node is trying to validate (in that scenario the node's public key isn't in the db yet because the block hasn't been pushed yet)

Steps to reproduce

unfortunately this is very hard to reproduce because we should need to wait for a smithing node to generate a spine block with a newly registered blocksmith (node). Then we would have to start a new non-smithing node and wait util it downloads that spine block. At that point the node wouldn't be able to validate the block and proceed downloading the blockchain.

Expected behavior

Spine blocks smithed with a valid spine public key shouldn't fail validation