implemented get_next

broker/CMakeLists.txt

broker/src/hidra2_broker/database/database.go

broker/src/hidra2_broker/database/database_test.go

broker/src/hidra2_broker/database/mock_database.go

broker/src/hidra2_broker/database/mongodb.go

broker/src/hidra2_broker/database/mongodb_test.go

broker/src/hidra2_broker/server/get_next.go

broker/src/hidra2_broker/server/get_next_test.go

broker/src/hidra2_broker/utils/helpers.go

broker/src/hidra2_broker/utils/status_codes.go

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/unittests/test_folder_to_db.cpp