Split bwauth children more equally
The bw scanners are currently divided into 4 sub-processes that each scan a fraction of the network.
However, the current file sizes are such that the subprocesses make progress unevently. Scanner 2 and 3 are about 2X faster than scanner 1, and scanner 4 is 2X faster than 2 and 3.
This ticket blocks on #1975 (moved), though.