Make methods of derived Workers final instead of virtual

[vweevers]

E.g. instead of:

leveldown/binding.cc

Lines 848 to 850 in 3ff48f1

	virtual void DoExecute () {
	SetStatus(database_->Put(options_, key_, value_));
	}

Write:

 void DoExecute () final { 
   SetStatus(database_->Put(options_, key_, value_)); 
 }

[ralphtheninja]

Iirc they are still virtual so this is why I kept the virtual keyword. But fine if you want to remove it.

[vweevers]

After reading a bit, I see why you'd want to keep virtual. It makes no difference to the compiler, but it's helpful information to humans. Without it you'd have to look at the base class to find a method is virtual.

So instead I want to:

1. Make structs final (rather than their methods); I now realize that's what I wanted to communicate
2. Replace virtual with the override keyword where appropriate (this still communicates the virtualness, and checks that the base class' method is virtual).