Better synchronization of CmdCallbackI subclasses

onFinished() was being called from the ctor of CmdCallbackI
due to the call to poll. This was before the "container"
field of ImportCallback had been setting meaning nothing
was available for processing. CountDownLatches have now
been added to the superclass to allow proper control of
ordering, and the call to poll now happens in a background
thread so that onFinished() does not block.