This was precipitated by OOZIE-2797. We had a case in production where the number of active metastore delegation tokens outstripped the ZooKeeper jute.maxBuffer size. Delegation tokens could neither be fetched, nor be cancelled.
The root-cause turned out to be a miscommunication, causing delegation tokens fetched by Oozie not to be cancelled automatically from HCat. This was sorted out as part of OOZIE-2797.
The issue exposed how poor the log-messages were, in the code pertaining to token fetch/cancellation. We also found need for a tool to query/list/purge delegation tokens that might have expired already. This patch introduces such a tool, and improves the log-messages.