Use RAII for resource management in QSslSocket (and its backends)

At the moment our QSslSocket's backends contain a lot of manual memory/resources management (it's not about new/delete, it's about platform/framework specific functions allocating some resource and the complement 'releasing' functions. This is very bug/error prone, every time somebody feels urge to introduce a new piece of code, they leak resources in a scenario like this:

if (somethingFailed) {
   qCWarning(lcSsl, "something failed");
   return; // no q_XXX_free()... etc.
} 

Also, we can use RAII in private backend classes (instead of naked pointers) wherever possible.