Filter out non-viable branches when call to get_head

[ColinSchwarz]

v0.9.3: filter out non-viable branches when call to get_head (ethereum/consensus-specs#1495)

[twoeths]

I suppose we have nothing to do with this because we use different algorithm.

• According to the spec, we want to filter blocks before traverse them to getHead.
• For our algorithm, we maintain bestTarget so we only get it in head (getHead). We don't traverse blocks so we don't need to filter.

can you confirm @wemeetagain ?

[twoeths]

I took some more time on this. The main point is to make sure the head has the same finalized & justified checkpoint to the store, so I propose:

• Maintain justified/finalized checkpoint in all nodes
• Whenever there is justified/finalized checkpoint change, we have to traverse from the justified checkpoint to decide new best target of every node (if needed)
• On weight change, make sure the best target has same justified/finalized checkpoint to the store.

[twoeths]

On the other hand, if we do like that it may not be worth to follow our own specific algorithm compared to the spec because we have to do the traversal anyway.