JetRecConfig: Safely unlock clonable children only

A rather nasty side-effect of the @clonable decorator used in JetRecConfig was leading to a situation where JetDefinition.clone() could unlock AthConfigFlags, which were held as a data member. The unlock of children should only be applied to other @clonable instances rather than anything under the sun.

For this to go wrong required a particular coincidence where:

1. I added a custom JetDefinition.__deepcopy__() that would not clone a new instance of the _cflags member
2. I was more scrupulous than usual about locking JetDefinition instances, leading to the built in clone() calls elsewhere performing the unlock

Discovering it required one more feature which was to call a function with @AccumulatorCache after the unlock happened, which would break on attempting to hash the unlocked flag container.

This fix is needed to make !70788 (merged) work safely and efficiently.

@delsart