Using Database::insert_multiple() instead of prepared statements speeds up trace import by a factor of 3-4. While being there, we now properly deal with nonexistent extended trace values (i.e., put NULLs into the DB). Side note: The ElfImporter should switch to insert_multiple(), too. Change-Id: I96785e9775e3ef4f242fd50720d5c34adb4e88a1