[ofiwg] Proposal for collective offload support

[Hefty, Sean]

I've opened a pull request that enhances the libfabric API to support fabric based collectives.  The proposal is based closely on the presentation covered in the OFIWG meetings, with the bulk of the changes being new man pages.

https://github.com/ofiwg/libfabric/pull/5053

This proposal is targeting a 1.9 release and would not be merged until after 1.8.

- Sean