added producer_impl tests

common/cpp/CMakeLists.txt

common/cpp/include/common/file_info.h

common/cpp/include/common/networking.h

common/cpp/include/common/os.h

common/cpp/include/system_wrappers/io.h

common/cpp/include/system_wrappers/system_io.h

common/cpp/src/system_io.cpp

common/cpp/src/system_io_linux.cpp

common/cpp/unittests/MockIO.h

producer/api/src/producer_impl.cpp

producer/api/src/producer_impl.h

producer/api/unittests/test_producer_impl.cpp

producer/dummy-event-detector-cpp/src/dummy_detector.cpp

producer/dummy-event-detector-cpp/src/dummy_detector.h

receiver/CMakeLists.txt

receiver/src/file_reference_handler.cpp

receiver/src/file_reference_handler.h

receiver/src/network_producer_peer.cpp

receiver/src/network_producer_peer.h

receiver/src/network_producer_peer_handlers.cpp