Consider storing measured bandwidth for both the measured relay and the helper relay

Currently sbws store the measurement for the relay that was selected to be measured, but probably the measurement should also be stored for the helper relay, which would reduce to the half the measurements needed.

The main difference would be that the helper measurement would be with a relay that have the same or half bandwidth instead of with a relay that have double or same bandwidth, which might restrict its bandwidth.

To mitigate this, it could be checked whether it was a helper and then measure it next time as the node to measure.

Different to legacy/trac#28463 (moved), though related.