Slow ADSL owners use case

[dmorn]

Make booster solve the use case were ADSL provided is not fast (or reliable) enough to cope with our network traffic.
booster already shows benefits (as shown by the speedtests made up to now) on this point using the "naive" round robin strategy, but we need more:

• fill sources without overcharging them
• consider unreliable sources
• solve Metrics: collect number of times a source is selected #21 and Strategy & Policies: make them work together #26