How long should sbws keep measured and observed bandwidths?

In legacy/trac#27135 (moved), sbws starts keeping observed bandwidths for relays:

Taking the descriptor observed bandwidth only when the relay is measured and calculating the mean when there're several observed bandwidth values for the same relay

Here are some options:

• use the latest measured and observed bandwidth
• take the latest measured and observed bandwidth every hour, and
  • average the last N days of bandwidths
  • apply an exponentially decaying average to all bandwidths

We need to decide which strategy to use, update the bandwidth file spec, and implement this feature in sbws.