Using INSERT DELAYED on MERGE tables could lead to table
corruptions.

The manual lists a couple of storage engines, which can be
used with INSERT DELAYED. MERGE is not in this list.

The attempt to try it anyway has not been rejected yet.
This bug was not detected earlier as it can work under
special circumstances. Most notable is low concurrency.

To be safe, this patch rejects any attempt to use INSERT
DELAYED on MERGE tables.


mysql-test/r/merge.result:
  Bug#26464 - insert delayed + update + merge = corruption
  Added test result.
mysql-test/t/merge.test:
  Bug#26464 - insert delayed + update + merge = corruption
  Added test.
sql/ha_myisammrg.h:
  Bug#26464 - insert delayed + update + merge = corruption
  Removed HA_CAN_INSERT_DELAYED flag from table_flags().
  The insert delayed thread upgrades the lock from the first
  entry in MYSQL_LOCK::locks only. Hence it is incapable to
  handle MERGE tables, which have as many entries in this
  array as they have MyISAM sub-tables.