(aka "deinit is not called when calling udf from trigger").

We should call udf_deinit() function during cleanup phase after prepared
(or ordinary) statement execution instead of calling it from Item's
desctructor.

No test case is provided since it is hard to test UDF's from our test
suite.


sql/item_func.cc:
  udf_handler:
   Moved all functionality from udf_handler::~udf_handler() to 
   udf_handler::cleanup() method which will be called after
   each PS execution, thus allowing udf_deinit() to be executed
   symetrically with udf_init() (which is executed for each
   execution of PS).
  Added Item_udf_func::cleanup() which performs proper cleanup after
  execution of PS with UDF function.
sql/item_func.h:
  Added Item_udf_func::cleanup() method to perform cleanup properly after
  execution of PS with UDF function.
sql/item_sum.cc:
  Added Item_udf_sum::cleanup() method to perform cleanup properly after
  execution of PS with aggregate UDF function.
sql/item_sum.h:
  Added Item_udf_sum::cleanup() method to perform cleanup properly after
  execution of PS with aggregate UDF function.
sql/sql_udf.h:
  Added udf_handler::cleanup() method declaration which is responsible
  for cleaning up UDF execution context at the end of execution of
  statement (using ~udf_handler() for this purprose did not worked for PS).