and Stored Procedure

The essence of the bug was that for every re-execution of stored
routine or prepared statement new items for character set conversions
were created, thus increasing the number of items and the time of their
processing, and creating memory leak.

No test case is provided since current test suite can't cover such type
of bugs.


mysql-test/r/sp.result:
  Add result for bug#21013: Performance Degrades when importing data
  that uses Trigger and Stored Procedure.
mysql-test/t/sp.test:
  Add test case for bug#21013: Performance Degrades when importing data
  that uses Trigger and Stored Procedure.
sql/item.cc:
  Switch arena only when in statement prepare mode.  Subsequent executions
  will use cached item tree.