Deprioritize files that have been recently queued

[StephenCWills]

• Add FileWatcher.DeprioritizationThreshold to system settings
• Save LastQueuedAtWatcherPriority to the file index
  • This will be saved in-memory, associated with the whole file group
  • The file index gets recreated in OnReconfigure(), so updating configuration in system settings will cause the system to forget LastQueuedAtWatcherPriority
  • WatchDirectoryBalancerNode invokes ReconfigureWatchDirectories() which bypasses OnReconfigure() so it thankfully does not recreate the index
• Default deprioritization threshold is 0, meaning it shouldn't deprioritize at all unless the system clock changes