refactoring statistics class

receiver/CMakeLists.txt

receiver/src/connection.cpp

receiver/src/connection.h

receiver/src/receiver_statistics.cpp

receiver/src/receiver_statistics.h

receiver/src/request.cpp

receiver/src/request.h

receiver/src/request_handler.h

receiver/src/requests_dispatcher.cpp

receiver/src/requests_dispatcher.h

receiver/src/statistics.cpp

receiver/src/statistics.h

receiver/src/statistics_sender_fluentd.cpp

receiver/src/statistics_sender_influx_db.cpp

receiver/unittests/receiver_mocking.h

receiver/unittests/test_connection.cpp

receiver/unittests/test_receiver_statistics.cpp

receiver/unittests/test_request.cpp

receiver/unittests/test_requests_dispatcher.cpp

receiver/unittests/test_statistics.cpp