Symmetric key generation

Fixed issue for new key server:
-new key is generated when keyserver changes
-Added unit test for verifying
Added support for changing symmetric key on every view change
-configurable through property 'change_keys'
Removed obsolete comment
Test case for symmetric key change
Cleaned up tests for ENCRYPT
ENCRYPT changes (https://issues.jboss.org/browse/JGRP-1760)