... This means unneeded block files can be reclaimed sooner.
This also eliminates the checks of use_count, which might vary in a
multithreaded context; though that didn't really matter (because once it
was below 2, the function had the only remaining shared_ptr to the block file,
so it could not have increased again to 2).
... for functions in final classes.
override is like const -- it's not necessary, but it helps the compiler to
catch mistakes.
There may be some overriding functions not explicitly declared virtual and I did
not identify such cases, in which I might also add override.
... Should have no effect on generated code, except perhaps some slight faster
virtual function calls. Mostly useful as documentation of design intent.
Tried to mark every one of our classes that inherits from another, or is a
base for others, or has abstract virtual functions, and a few others besides.
