Dynamic JAAS change for clients

Clients using JAAS based authentication are now forced to restart themselves in order to reload the JAAS configuration. We could

• make the sasl.jaas.config dynamically configurable and therefore better equip them to changing tokens etc.
• detect file system level changes in configured JAAS and reload the context.

Original issue:
Re-authentication feature on broker side is under implementation which will enforce consumer/producer instances to re-authenticate time to time. It would be good to set the latest delegation token dynamically and not re-creating consumer/producer instances.