v3.1.0

The S3 backend's startup removal of orphaned bucket directories can now be told to leave a bucket directory alone: create a directory with the same name in CMGR_ARTIFACT_DIR and put an empty .dont-purge file in it. For example, $CMGR_ARTIFACT_DIR/other-stuff/.dont-purge keeps other-stuff/ in the bucket (under path-prefix, if one is set). This lets a bucket be shared with files this server did not publish without turning the removal off (-o prune-orphans=false) for everything else. See orphan removal.