N 4321 : Towards Implementation and Use of memory order consume Doc

This document is a revision of WG21/N4321, based on email discusssions and including yet another proposal in Section 7.9. This proposal has been further refined by discussions on various email reflectors. WG21/N4321 is itself a revision of WG21/N4215, based on feedback at the 2014 UIUC meeeting and on the various email reflectors. WG21/N4215 is in turn a revision of WG21/N4036, based on feedback at the 2014 Rapperswil meeting, at the 2014 Redmond SG1 meeting, and on the various email reflectors. A detailed change log appears starting on page 38.