Add file to allow operator manual kill of transfers

I reused code from Hassen from an old pull request (dmwm/CRABServer#4036). The code changes the state of documents in CouchDB to 'killed' using the updateDocument() method from class Database.
The script should be run using the manage script in dmwm/deployment/asyncstageout:
manage execute-asyncstageout kill-transfer -t <taskname>