This patch adds messages for the "purge histories" step:
1. A message is printed when processing a new history (so that the user can tell to which history the following dataset/hda messages belong).
2. in "info_only" mode, a message is printed about each deleted history
3. in "info_only" mode, a message is printed for each HDA that is checked
4. The message for HDA is changed to include the associated dataset id.