Add network backend with libfabric

[mcopik]

We want to extend rFaaS with support for Cray interconnects. The proper way would be to add a libfabric implementation with the ugni provider.

• [ ] Isolate ibverbs/rdmacm network code and define network lib interface.
• [x] Initialize ugni with the help of cray-mpi
• [ ] Implement basic P2P communication.
• [ ] Implement and test atomic operations.
• [x] Evaluate communication between processes from different SLURM jobs (credentials exchange).
• [ ] Extend test suite.