sbws: weight bandwidths based on the time since the last bandwidth

Split off legacy/trac#27346 (moved):

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