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.
• Initialize ugni with the help of cray-mpi
• Implement basic P2P communication.
• Implement and test atomic operations.
• Evaluate communication between processes from different SLURM jobs (credentials exchange).
• Extend test suite.