CDH-20513: Crash during vcore acquisition callback

This patch fixes a bug where the RM vcore acquisition thread might call
into HdfsScanNode::ThreadTokenAvailableCb() after the scan node has been
destroyed. Now, during Close() the scan node removes the callback it had
previously registered. This ensures that the callback will never be
invoked after Close() completes.

Change-Id: Icb50c14a45c9c67a52ecb6e8a3324b0721a5a431
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3889
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins