This patch fixes the architectural problem that was in the previous one:
* class handler is not anymore abused with extra methods that are needed only
locally during update, and belong to sql layer.

* stack is not abused with extra thread_local variables.

The prototype the function must follow now is:
int (*)(T*, const uchar*, const uchar*);

A pointer to member is converted into a function pointer. For that we need to
extract the correct one from vtable. See rtti.h. vtable and pointer to member
formats are implementation-defined, so RTTI::Method_intrn will have to be
implemented for every abi.

The two const uchar* arguments are still excessive for some cases. This only can
be improved with true JIT code generation.

class Exec_plan (exec_plan.h) handles storing, modifying and executing the plan.
class Update_execution_plan hold all the members to call and all the parameters
needed.

Update_execution_plan replaces some variables in update_single_table. It lies on
stack so the performance will be the same.