Guardfraction on dirauths screws up bandwidth weights

It seems that dirauths stopped including bandwidth weights on the consensus after Guardfraction got enabled: https://lists.torproject.org/pipermail/tor-dev/2015-June/008908.html

Looking at weasel's dirauth we get plenty of related error messages:

[warn] Bw Weights error 1 for Case 3be (E scarce, Wee=1, Wmd == Wgd) v10. G=14793673 M=8310679 E=3428814 D=7040272 T=26698303 Wmd=-512 Wme=0 Wmg=2192 Wed=11025 Wee=10000 Wgd=-512 Wgg=7808 Wme=0 Wmg=2192 weight_scale=10000

That is, it seems like the bandwidth calculation of the dirauths got screwed a bit. This might be the result of using the guardfraction data during bandwidth calculation in compute_weighted_bandwidths() that might combine badly with #13297 (moved).

The negative Wmd/Wgd values seem weird here as well.

Let's disable the GuardFraction feature for now, till we figure out this issue.