Final added to:
- All reasonable classes inhereted from Field
- All classes inhereted from Protocol
- Almost all Handler classes
- Some important Item classes

The stripped size of mariadbd is just 4K smaller, but several object files
showed notable improvements in common execution paths.
- Checked field.o and item_sum.o

Other things:
- Added 'override' to a few class functions touched by this patch.
- Removed 'virtual' from a new class functions that had/got 'override'
- Changed Protocol_discard to inherit from Protocol instad of Protocol_text