Periodic

Make the timelapse image requests strictly periodic at the requested time.

The original code took a picture, then waited the requested time, then took the next picture.  Because it takes time to take the picture, and because that time is variable (since autoexposure etc. take a few pictures to settle down) the time-lapse is irregular.

Note that this pull request incorporates my previous pull request (--latest image and preventing partial image files) because I don't know how to split them out.