[Core] Add TClass::GetClass<T> method to lower contention

The method TClass::GetClass<T> obtains the TClass instance pointer
corresponding to T in two ways, according to the nature of T.
If T inherits from or is TObject, the TClass* value is obtained via the
T::Class() static method, which locks only upon the first call.
If T is a generic type, internally the TClass::GetClass(type_info, ...)
method is called.

The new functionality is put to a good use by this commit in TBuffer
and TDirectoryFile.

It is worth noting how the usage of tag dispatching could be replaced
by 'if constexpr'.