ZOOKEEPER-3711: Dispose SaslServer instances after use

[ztzg]

Without this patch, the SaslServer instance held a ServerCnxn is not explicitly disposed() when the connection is closed. This means that we are relying on the GC finalizer to release associated resources.

While this does not seem to be problematic in practice, it is better to explicitly dispose() the object at close() time, as we know that we are not going to need it anymore. This is unlikely to make a difference for managed providers, but -Dsun.security.jgss.native=true can potentially change the game.