The INSERT DELAYED should not maintain its own private auto-increment
counter, because this is assuming that other threads cannot insert
into the table while the INSERT DELAYED thread is inserting, which is
a wrong assumption.

So the start of processing of a batch of INSERT rows in the 
INSERT DELAYED thread must be treated as a start of a new statement
and cached next_insert_id must be cleared.


mysql-test/r/delayed.result:
  test suite for the bug
mysql-test/t/delayed.test:
  test suite for the bug
sql/sql_insert.cc:
  Reset auto-increment cacheing before processing
  the next batch of inserts in the handler thread