Simplify whatToTrace / LL::NG unique ID management
After discussions, we agree that the whatToTrace configuration is not very clear.
We suggest to keep whatToTrace parameter, but instead of beeing just a reference to a session key, it will be an expression that can be evaluated, so we don't need no more the _whatToTrace macro.
We also need to rename this parameter. Propositions for now are: lemon_uid, ll_uid, llng_uid.