Improve sbws bandwidth accuracy

Better designs SHOULD:

• use at least 4 measurements that are at least 6 hours apart, because:
  • there is a daily cycle
  • each day contains 2 similar points in the cycle (it is an up and down cycle)
  • if all 4 measurements happen within a few hours, they will still be biased
• use at least 3 days of observed bandwidths, because:
  • a single download at the changeover point can affect 2 days
• weight bandwidths based on the time since the last bandwidth, because:
  • if we only record bandwidths when they change, bandwidths that are updated soon after the last bandwidth are weighted too high
  • we can either:
    • record the bandwidths every hour, even if they haven't changed
    • weight each bandwidth by the time since the last bandwidth
• use a decaying average for measured and observed bandwidths, because:
  • recent bandwidths are closer to the relay's current capacity
  • and we want accurate results