fix segfault with callback as class member in send_stream_finished_flag

common/cpp/include/asapo/request/request.h

common/cpp/src/request/request_pool.cpp

common/cpp/unittests/request/test_request_pool.cpp

producer/api/cpp/src/producer_request.h

producer/api/python/asapo_producer.pyx.in

tests/automatic/producer/python_api/check_linux.sh

tests/automatic/producer/python_api/check_windows.bat

tests/automatic/producer/python_api/producer_api.py