Improve UGI debug log to help troubleshooting TokenCache related issues

We have seen some issues around TokenCache getDelegationToken failures even though the UGI already has a valid token. The tricky part is the token map is keyed by the canonical service name, which can be different from the actual service field in the token, e.g. KMS token in HA case. The current UGI log dumps all the tokens but not the keys of the token map. This ticket is opened to include the complete token map information in the debug log.