- Changing sp_rcontext::m_var_items from list of Item to list of Item_field
- Renaming sp_rcontext::get_item() to get_variable() and changing
  its return type from Item* to Item_field *
- Adding sp_rcontext::get_parameter() and sp_rcontext::set_parameter(),
  wrappers for get_variable() and set_variable() with extra DBUG_ASSERT.
  Using new methods instead of get_variable()/set_variable() in
  relevant places.