add unit tests for parallel tasks

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

common/cpp/include/database/database.h

common/cpp/src/database/CMakeLists.txt

common/cpp/src/database/database.cpp

common/cpp/src/database/mongodb_client.cpp

common/cpp/src/database/mongodb_client.h

tests/mongo_db/connect/CMakeLists.txt

tests/mongo_db/connect/connect_mongodb.cpp

tests/mongo_db/insert/CMakeLists.txt

tests/mongo_db/insert/insert_mongodb.cpp

worker/tools/folder_to_db/CMakeLists.txt

worker/tools/folder_to_db/src/folder_db_importer.cpp

worker/tools/folder_to_db/src/folder_db_importer.h

worker/tools/folder_to_db/src/main.cpp

worker/tools/folder_to_db/unittests/test_folder_to_db.cpp