Fix RCTP multi-inheriting in Serialized and derived classes

This patch fixes Serialized derived classes, using CRTP idom,
by fixing the "constructor forwarding problem".

A derived class has no to use dht::Value::Serialized<Derived, Base>
as super class, by replacing itself in "Derived" and use the
right "Base" class. Ex:

class MyImMessage : public dht::Value::Serializable<MyImMessage, dht::ImMessage>
{
  ...
}

No more needed to overload getType and define TYPE static.