on a BLACKHOLE table

Using INSERT DELAYED on BLACKHOLE tables could lead to server
crash.

This happens because delayed thread wants to upgrade a lock,
but BLACKHOLE tables do not have locks at all.

This patch rejects attempts to use INSERT DELAYED on MERGE
tables.


mysql-test/r/blackhole.result:
  A test case for BUG#27998.
mysql-test/t/blackhole.test:
  A test case for BUG#27998.
sql/ha_blackhole.h:
  Removed HA_CAN_INSERT_DELAYED flag from table_flags().
  The insert delayed thread upgrades the lock. Hence it is
  incapable to handle BLACKHOLE tables, which do not have
  locks at all.