Pass parent wrapper directly instead of obfuscation key. This
makes it possible for other databases which re-use this code
to use other properties from the database.
Add a namespace dbwrapper_private for private functions to be used
only in dbwrapper.h/cpp and dbwrapper_tests.
Using throw() specifications in function signatures is not only
not required in C++, it is considered deprecated for
[various reasons](https://stackoverflow.com/questions/1055387/throw-keyword-in-functions-signature).
It is not implemented by any of the common C++ compilers. The usage is
also inconsistent with the rest of the source code.
