DM-27492: Improvements to pipetask execution options

[andy-slac]

Several improvements in pipetask execution options:

• New option --skip-existing-in which takes collection names(s), if output
  datasets already exist in those collections corresponding quanta is skipped.
• A --skip-existing option is now equivalent to appending output run
  collection to the --skip-existing-in list.
• An --extend-run option implicitly enables --skip-existing option.
• A --prune-replaced=unstore option only removes regular output datasets;
  InitOutputs, task configs, and package versions are not removed.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[timj]

Looks okay. I wasn't really able to get my head around all the test changes in a reasonable amount of time.