Merge compare-and-swap example and functionality into HICR branch

[KADichev]

While BSC collaboration had an example of compare-and-swap atomics, examples testing and demonstrating these were not in the repo. Here, we provide:

• new LPF functions lpf_lock_slot and lpf_unlock_slot, which are BLOCKING and lock and unlock global LPF slots. These are implemented in src/MPI/ibverbs.cpp via opcode IB verbs atomic operation compare and swap. The lock slot expects a 0 and swaps it to 1. The unlock slot expects a 1 and swaps it to 1. In both cases, if the expected value is not found, we keep posting and polling until we find the value we expect. Hence, both of these calls are blocking, in contrast to much of the lpf semantics (such as put and get calls).
• The example testing this makes every rank (including 0) get an exclusive lock on a global slot held at rank 0, increment a global counter held at rank 0 by 1, and release the lock. In the end, we expect the rank 0 to hold the number of ranks in its global counter.