[Minor] change modifier of DefaultPrivilegeHandler.nextSessionId()

set method DefaultPrivilegeHandler.nextSessionId() to protected from
private so it can be used in sub classes

src/main/java/ch/eitchnet/privilege/handler/DefaultPrivilegeHandler.java

@@ -1018,7 +1018,7 @@ public class DefaultPrivilegeHandler implements PrivilegeHandler {
 	/**
 	 * @return a new session id
 	 */
-	private synchronized String nextSessionId() {
+	protected synchronized String nextSessionId() {
 		return Long.toString(++this.lastSessionId % Long.MAX_VALUE);
 	}