EHN: Add locking to async methods

include/caosdb/file_transmission/download_request_handler.h

include/caosdb/file_transmission/upload_request_handler.h

include/caosdb/logging.h

include/caosdb/transaction.h

include/caosdb/unary_rpc_handler.h

src/caosdb/entity.cpp

src/caosdb/file_transmission/download_request_handler.cpp

src/caosdb/file_transmission/upload_request_handler.cpp

src/caosdb/transaction.cpp

src/caosdb/transaction_handler.cpp

src/caosdb/unary_rpc_handler.cpp

test/test_transaction.cpp