fabtests/efa/multi_ep_stress: Refactor completion handling with dedicated CQ threads

[alekswn]

Refactor the multi-endpoint stress test to use dedicated completion queue (CQ) polling threads instead of inline polling. This change improves scalability and reduces contention when handling completions.

Key changes:

• Introduce cq_context structure to encapsulate CQ state with atomic completion counters and dedicated polling thread
• Replace blocking wait_for_comp() with lock-free atomic-based completion tracking that runs in separate threads
• Implement ft_backoff() with progressive backoff strategy (CPU pause, yield, exponential sleep) to reduce busy-waiting overhead
• Remove shared CQ mutex locking in favor of lock-free atomic operations
• Move CQ thread lifecycle management into setup_endpoint() and cleanup_endpoint()
• Refactor sender/receiver workers to poll atomic completion counters instead of directly calling fi_cq_read()
• Fix timeout handling to distinguish between -FI_ETIMEDOUT and other errors
• Move shared resource setup/cleanup to run_test() for better lifecycle management

This design eliminates lock contention on shared CQs and allows worker threads to continue posting operations while a dedicated thread handles completions asynchronously.