[Sim] Update Multisim to Use Concurrency Instead of Async

[trizin]

Currently, the multisim uses asyncio for handling multiple runs concurrently. While asyncio is effective for I/O bound operations, it still runs on a single thread and is not well-suited for CPU-intensive tasks. To improve the performance of multisim, update the implementation to use multithreading.

[trentmc]

Priorities have mostly shifted away from pdr-backend. So closing less-critical issues.