Fix reference leak of subscription callbacks between Python and Java
Originally this MR:
- Implemented a unit test to demonstrate garbage collection issue from https://issues.cern.ch/browse/ACCPY-520.
Subsequently:
- Fix implemented. It depends on improved reference counting in JPype 0.7 compared to 0.6. Fundamentally the issue identified in https://github.com/jpype-project/jpype/pull/581 has to be worked around manually as per this merge request.