some more refactoring

3d_party/mongo-c-driver/install.sh

README.md

common/cpp/src/json_parser/rapid_json.cpp

consumer/tools/folder_to_db/src/main.cpp

...site/versioned_examples/version-21.06.0/python/produce.py

receiver/CMakeLists.txt

receiver/README.md

receiver/src/connection.cpp

receiver/src/main.cpp

receiver/src/receiver.cpp

receiver/src/receiver_config.cpp

receiver/src/receiver_config.h

receiver/src/receiver_config_factory.h

receiver/src/statistics/statistics.h

receiver/src/statistics/statistics_sender_fluentd.cpp

receiver/unittests/mock_receiver_config.cpp

receiver/unittests/statistics/test_receiver_statistics.cpp

receiver/unittests/statistics/test_statistics.cpp

...r/unittests/statistics/test_statistics_sender_fluentd.cpp

...unittests/statistics/test_statistics_sender_influx_db.cpp