Support Kerberos authentication of clients.
The following usage would let an admin use Kerberos authentication to assign ACLs to authenticated clients.
1. Admin logs into zookeeper (not necessarily through Kerberos however).
2. Admin decides that a new node called '/mynode' should be owned by the user 'zkclient' and have full permissions on this.
3. Admin does: zk> create /mynode content sasl:zkclient@FOOFERS.ORG:cdrwa
4. User 'zkclient' logins to kerberos using the command line utility 'kinit'.
5. User connects to zookeeper server using a Kerberos-enabled version of zkClient (ZookeeperMain).
6. Behind the scenes, the client and server exchange authentication information. User is now authenticated as 'zkclient'.
7. User accesses /mynode with permissions 'cdrwa'.