Limit the in-flight downloads for model parameters

[IAvecilla]

This PR introduces a download scheduler for concurrency control and structured retry logic for P2P parameter sharing:

• New DownloadSchedulerHandle actor that manages concurrent download capacity. It enforces a configurable max concurrent downloads limit.
• Unified retry logic with differentiated policies:
  • Model paramaters and config: Retries immediately with no max retry limit. Retries are gated by the concurrency limiter, so they only start when a slot is available (max 8 at the sime time).
  • Distro results: Exponential backoff (backoff_base * 2^retries) with a configurable max retry count (default 3). These are not gated by download capacity.
• Moved download_manager.rs into a download/ module with manager.rs and scheduler.rs.
• Add unit tests for the scheduler covering capacity management, FIFO ordering, retry policies, and edge cases