client: Reorg to new best when finalizing divergent branch (#2869)

* client: add tests for reorging on diverged finality

* client: mark finalized block as best if diverged from current best chain

* client: update meta on set_head

* core: add docs about SelectChain to finalize_block

* client: improve finality reorg test

* client: LongestChain doesn't return client best block

* client: LongestChain searches canonical chain