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