API for atomic operations

[mchaarawi]

Several network providers provide direct hardware support for some atomic operations like test_and_set, fetch_and_op, compare_and_swap, etc. It would be nice to have those operations exposed from OFI through the mercury API. I found this man page useful to summarize a few of those operations that are provided by OFI: https://manpages.debian.org/testing/libfabric-dev/fi_fetch_atomic.3.en.html

We can use that to implement an atomic counter in DAOS for example that would be useful for HDF5 and other middleware.