Update scoring function for importance score

[ChristopherHogan]

• [ ] Add configurable weights for each variable.
• [ ] variables
  • recency
  • frequency
  • size
  • Bucket reference count
  • VBucket link count
• [ ] Make ~5 "buckets" for each variable so we can normalize the importance score to [0, 1].
• [ ] Add a policy for each variable (config param) that adjusts the weights towards that variable
• [ ] Describe the importance scoring function in the wiki.