Bandwidth auths should publish average and weighted onionskin failure rates
While testing for path bias rates, I've noticed that the onionskin failure rate in the network can vary as much as +/- 30% in the matter of a few hours. This is rather insane.
We should publish statistics on this failure rate over time somehow. This might be a metrics task, but if the bandwidth auths do it, we can have them set consensus parameters for the path bias code to decide on how loudly to scream and when (see Proposal 209).
For that, I think I want network-wide averages, as well as consensus-bandwidth weighted averages. The path bias levels should be set using consensus-bandwidth weighted values.
See also legacy/trac#5457 (moved), but I don't think we should do that anymore, based on how easy it seems to be to cause onionskin overload.