GeneratorObjectsTPCnv:+AthAllocators Fix DataPool usage for HepMC3.

With HepMC3, when we make a shared_ptr from an object we're just allocated from a DataPool, we need to ensure that there are no other shared_ptr's referencing that object; otherwise the behavior is undefined (but can manifest as the weak references in GenParticle mysteriously expiring; see ATR-26790).

Set up clear hooks on the DataPools so that the objects get cleared when they're returned to the pools.