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

substrate/core/client/db/src/lib.rs

@@ -1118,6 +1118,7 @@ impl<Block: BlockT<Hash=H256>> Backend<Block> {
 					hash.clone(),
 					(number.clone(), hash.clone())
 				)?;
+				meta_updates.push((hash, *number, true, false));
 			} else {
 				return Err(client::error::Error::UnknownBlock(format!("Cannot set head {:?}", set_head)))
 			}