Bug#20627 - INSERT DELAYED does not honour auto_increment_* variables

INSERT DELAYED ignored an explicitly set INSERT_ID and session
specific auto_increment_* variables.

The problem was that the inserts are done by a system thread,
which does not have access to the session variables of the user
thread.

On a proposal of Guilhem I fixed it so that the variables are
copied to the data structure for every delayed row. The system
thread sets its session variables from these values.


mysql-test/r/delayed.result:
  Bug#20830 - INSERT DELAYED does not honour SET INSERT_ID
  Bug#20627 - INSERT DELAYED does not honour auto_increment_* variables
  Turned some sleeps into FLUSH TABLEs.
  Added test cases.
mysql-test/t/delayed.test:
  Bug#20830 - INSERT DELAYED does not honour SET INSERT_ID
  Bug#20627 - INSERT DELAYED does not honour auto_increment_* variables
  Turned some sleeps into FLUSH TABLEs.
  Added test cases.
sql/sql_insert.cc:
  Bug#20830 - INSERT DELAYED does not honour SET INSERT_ID
  Bug#20627 - INSERT DELAYED does not honour auto_increment_* variables
  Added auto_increment/insert_id related variables to 'delayed_row'.
  The session values are copied to 'delayed_row' by the user thread.
  The delayed insert thread copies them to his session variables.