The leak occurred because tracker_sparql_task_new_with_sparql() was being
called but the returned TrackerTask* was not being unreferenced anywhere and
the call to tracker_sparql_buffer_flush() with the new task was taking its own
references internally.

Took this opportunity to make the code here easier to follow:
- do_process_file() is now merged into item_add_or_update()
- item_add_or_update_cb() is renamed to item_add_or_update_continue() so it's
  obvious it is called from tracker_miner_fs_file_notify().
- renamed various variables to make the code easier to follow.