If a user starts a long-running HFTP client using a keytab, we should do relogin automatically whenever TGT expires. Currently, HFTP client uses TGT to fetch a delegation token and cache that delegation token for HFTP operations. The delegation token is automatically renewed/refetched using TGT. However, when TGT expires, delegation token renewal/refetch will fail and no further HFTP operation is possible. This is unsatisfactory since the user has given us her keytab. We should be able to relogin from keytab and continue.