Added 'records_out' and join_type to POSITION

records_out is the numbers of rows expected to be accepted from a table.
records_read is in contrast the number of rows that the optimizer excepts
to read from the engine.

This patch causes not plan changes. The differences in test results comes
from renaming "records" to "records_read" and printing of record_out in
the optimizer trace.

Other things:
- Renamed table_cond_selectivity() to table_after_join_selectivity()
  to make the purpose of the function more clear.

mysql-test/main/opt_trace_index_merge.result

@@ -226,7 +226,8 @@ explain select * from t1 where a=1 or b=1	{
                       ],
                       "chosen_access_method": {
                         "type": "index_merge",
-                        "records": 2,
+                        "records_read": 2,
+                        "records_out": 2,
                         "cost": 2.601171589,
                         "uses_join_buffering": false
                       }