Fixed clang -Winconsistent-missing-override warning.

Fixed clang -Winconsistent-missing-override warning.

AsyncJob classes can now use C++11 overrides as long as they use the new
CBDATA_CHILD() macro instead of old CBDATA_CLASS().

I have prohibited multiple CBDATA_CHILD() classes on the same
inheritance branch by adding the "final" specifier to toCbdata(). Such
classes feel dangerous because they may have different sizes and it is
not obvious to me whether the cbdata code will call the right size-
specific delete for them. We can easily relax this later if needed.