make min_bw configurable in bwauths
In a testing network, there isn't much traffic. So skipping nodes with an observed bandwidth less than 1 MByte/second means that it's easy to skip a large fraction of the network.
It would be helpful if min_bw were a configurable option in bwauthority.cfg. If it is an option, we could consider making the unmeasured node scanner measure all nodes, regardless of bandwidth.