WL#3601: Stored Procedures: rewrite Item_func_sp::sp_result_field

This is a refactoring worklog entry.

Since WL#2984 was implemented, the existing implementation of
Item_func_sp::sp_result_field became obsolete. It is slow, as a new field is
created for every invocation, and more importantly, it is error prone (see
BUG#23561 for details).

Unfortunately, sp_result_field() can not be rewritten by using
sp_head->sp_rcontext object, because sp_result_field() is called
when sp_rcontext instance does not exist yet. There are the following
ideas about possible solution:
  - use sp_result_field() by sp_rcontext somehow;
  - create a field for RETURNS-clause somewhere else and use it
    in sp_rcontext and in sp_result_field();
  - create sp_rcontext instance on demand.