types: sort validators by voting power

thus enabling faster commit verification assuming non uniform distribution of power.

Closes #2478

spec: https://github.com/tendermint/spec/pull/91

lite2/helpers_test.go

@@ -78,7 +78,7 @@ func (pkz privKeys) signHeader(header *types.Header, first, last int) *types.Com
 	}
 
 	// We need this list to keep the ordering.
-	vset := pkz.ToValidators(1, 0)
+	vset := pkz.ToValidators(1, 1)
 
 	blockID := types.BlockID{
 		Hash:        header.Hash(),