OPENDJ-2650 No more task log (detail or error) when running tools in online mode (import, backup,...)

The problem is linked to the removal of if (attributeType.mayHaveSubordinateTypes()) checks in Entry.java.
When it was there, this check ensured the method Entry.getAttribute(AttributeType)
always returned the internal List representing the ds-task-log-message attribute.
Thus the code in Task.addLogMessage() was always updating the internal state of the task entry.

Now that we have removed the aforementioned check, Entry.getAttribute(AttributeType)
always returns a newly created List object, thus the update performed by Task.addLogMessage() is lost.

Task.java:
In addLogMessage(), used a more robust way to update the ds-task-log-message attribute value.