Add scan concept

Most of the time, Lima acquisitions are part of a broader concept (e.g. scan, sequence). It would be interesting to make Lima aware of this concept (let's use scan) and to provide:

• A way to set a scan_uid. If none/empty (default), keep the old behavior;
• If a scan_uid is set, ImageStatus counters (aka last_image_xxx) would be reset only when the scan_uid changes;
• Provide scan_uid in the frame DevEncoded header returned by readImage();
• Overload readImage([frame_idx, scan_uid]) to ensure that we get an image (eventually the latest image with -1) of a specific scan (would return None if a new scan has been prepared).

AFAIU, it would remove a lot of complexity in the clients code (e.g. blissdata, flint).

cc: @valentin.valls, @matias.guijarro