Feature/improved collation cancel

[drmick]

• Removed CancelPending and simplified collator state management to Active/Waiting/Cancelled.
• Added a new command_complete notify signal from collator to manager.
• Implemented cancel_active_collators(...) in the manager: it marks active collators as cancelled, sends graceful cancel, and waits for the current
  command to finish.
• Updated mismatch/sync paths to explicitly cancel active collators and wait before running sync.
• Treated ExternalCancel as an internal graceful-cancel path (no regular on_cancelled dispatch handling).