IRAC metrics

Add the following metrics to IRAC:

• Queue size
• Number of conflicts (3 counters?)
  • merge policy discard update (local value wins)
  • merge policy applies update (remove value wins)
  • merge policy created a new value (merge remove value with local value)
• Merge policy run time (user code? warning if it takes too long?)
• Number of discards (old version updates)