JGRP-1838 & JGRP-1839 SASL fixes 

JGRP-1838 SASL separate CallbackHandler depending on mode of operation
JGRP-1839 Wrap calls to saslClient.evaluateChallenge in a PrivilegedAction for GSSAPI

https://issues.jboss.org/browse/JGRP-1838
https://issues.jboss.org/browse/JGRP-1839