In preparation for implementing custom aggregate functions, refactor
the common code between regular stored functions and aggregate stored
functions. This includes:

* initialising SP result field
* executing a SP
* access checks

In addition, refactor sp_head::execute_function to take two extra
parameters, a function rcontext and a Query_arena. These two paremeters
were initially initialised and destroyed within
sp_head::execute_function, but for aggregate stored functions we will
require control over their lifetime. The owner of these objects now
becomes Item_sp.
Signed-off-by: Vicențiu Ciorbaru <vicentiu@mariadb.org>