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.