Fix concurrency issue in BOLDStateProvider.CollectMachineHashes method.

The method was using s.Lock() which blocks all other operations on the BOLDStateProvider, including other read operations.

This creates a severe performance bottleneck in the BOLD protocol where multiple Edge Trackers can call CollectMachineHashes concurrently.

Changes:

• Replace s.Lock() with s.RLock() in CollectMachineHashes method
• This allows multiple concurrent read operations while maintaining thread safety
• Aligns with the BOLD protocol's requirement for concurrent challenge participation