Reorg best chain on finality if diverged

[gnunicorn]

If we are told a new block is finalized, that isn't a direct ancestor of what we currently considered the best chain, we need to look back to find the best common ancestor and reorganize our chain accordingly.

Starting point:

substrate/core/client/src/client.rs

Line 719 in a2dfec4

if route_from_best.common_block().hash != block {