Kerberos Auth: keep ticket and renew it periodically instead of creating always a new ticket

[nicoloboschi]

JIRA: https://dev.blazingcache.org/jira/browse/BLAZ-71

Reporter: eolivelli @eolivelli

This improvement will be rellay useful on the server-side.

See apache/bookkeeper@667390d
for a similar implementation on Apache BookKeeper which in turn is taken from Apache ZooKeeper