[ofiwg] API for Libfabric Overrides

[Hefty, Sean]

> Thanks for the feedback. I'll incorporate the feedback and open a proposal PR.

I have a patch set under development that will make use of fi_set_ops() in order to optimize interactions between the rxm and tcp providers.  There's also an existing use of fi_get_ops() to communicate between rxm and verbs.  It's likely other uses of get/set ops could occur to setup communication between different providers working together.

- Sean