Do not refresh again when the watcher sees a file the update service wrote itself #170

When update_from_memory or a polled update writes a data file kept on disk, last_changed_time is not moved. So with the file system watcher on, its next pass (track_local_data_file_changes in pipeline/fiftyone_pipeline_engines/src/fiftyone_pipeline_engines/datafile_update_service.py) sees the new time, refreshes the engine a second time and reports that to the update complete callbacks, which an update from memory must not do (data-updates.md lines 372 to 376 at 706de1f0).

Done when the service moves last_changed_time to the time of the file it wrote, and a test with the watcher on shows one refresh and no report after an update from memory.

Found by #169.