Adjust cost for re-creating a row from the JOIN CACHE

Creating a record from the join cache is faster than getting a row from
the engine (less and simpler code to execute).

Added JOIN_CACHE_ROW_COPY_COST_FACTOR (0.5 for now) as the factor to
take this into account. This is multiplied with ROW_COPY_COST.

Other things:
- Added cost of copying rows to hash join, similar to join_cache joins.

mysql-test/main/status.result

@@ -173,7 +173,7 @@ a	a
 1	1
 SHOW SESSION STATUS LIKE 'Last_query_cost';
 Variable_name	Value
-Last_query_cost	4.003418
+Last_query_cost	3.953418
 DROP TABLE t1;
 connect  con1,localhost,root,,;
 show status like 'com_show_status';