Update SaslClientCallbackHandler.java : Support for callbackhandler to be utilized directly by jgroups.xml <SASL/> block configuration.

Add default constructor
Changed declaration of client_password and client_name to private from private final.
Added getters/setters for client_password and client_name for supporting the instantiation of instances via new default constructor.

I have implemented callbackhandlers for use with node authentication work for a client, and discovered that it was not possible to 1) Use the refeferenced client_callbackhandler with the pre-conditions depicted in the iff statement in SASL.java; 2) If a block within an incident jgroups configuration descriptor attempts to use the built-in SaslClientCallbackHandler, a classnotfound type of exception occurs.

After some discussion with Tristan, I discovered that a default constructor and relevant properties setter/getter are necessary for property access to client_password, and client_password.