Minor fixes + cleanup.

transfer_service/abort_job_rpc_server.py

transfer_service/checksum.py

transfer_service/data_rpc_server.py

transfer_service/group_rw_executor.py

transfer_service/group_rw_rpc_server.py

transfer_service/import_executor.py

transfer_service/import_rpc_server.py

transfer_service/job_rpc_server.py

transfer_service/log_listener.py

transfer_service/retrieve_cleaner.py

transfer_service/retrieve_executor.py

transfer_service/retrieve_preprocessor.py

transfer_service/start_job_rpc_server.py

transfer_service/storage_rpc_server.py

transfer_service/store_executor.py

transfer_service/store_preprocessor.py

transfer_service/system_utils.py

transfer_service/tape_client.py