stop producer threads at python exit

common/cpp/include/request/request_pool.h

common/cpp/src/request/request_pool.cpp

producer/api/cpp/include/producer/producer.h

producer/api/cpp/src/producer_impl.cpp

producer/api/cpp/src/producer_impl.h

producer/api/cpp/unittests/test_producer_impl.cpp

producer/api/python/asapo_producer.pxd

producer/api/python/asapo_producer.pyx.in

producer/api/python/asapo_wrappers.h