Revisit MODCLUSTER-337 & MODCLUSTER-338

Revisits these two:

https://issues.jboss.org/browse/MODCLUSTER-337
https://issues.jboss.org/browse/MODCLUSTER-338

The original fix was to not send the message digest if the security key was not configured. However, this breaks backward compatibility a little.

I am now proposing a better solution: the digest that 1.2.x native part always sends is computed from zero byte array, thus if the security key is not configured, the Java side will use that for verification. So now all messages are checked for digest.

Also adds trace logging so its easier to debug.