There's currently no way of knowing the determinicity of an UDF.
And the optimizer and the sequence() UDFs were making wrong
assumptions about what the is_const member means.
Plus there was no implementation of update_system_tables()
causing the optimizer to overwrite the information returned by
the <udf>_init function.

Fixed by equating the assumptions about the semantics of 
is_const and providing a implementation of update_used_tables().
Added a TODO item for the UDF API change needed to make a better 
implementation.


include/mysql_com.h:
  Bug #30355: comment added
mysql-test/r/udf.result:
  Bug #30355: test case
mysql-test/t/udf.test:
  Bug #30355: test case
sql/item_func.cc:
  Bug #30355: keep const_item_cache and used_tables_cache in sync
sql/item_func.h:
  Bug #30355: 
   - a better implementation of update_used_tables()
   - keep const_item_cache and used_tables_cache in sync
sql/udf_example.c:
  Bug #30355: Wrong value for const_item fixed.