Investigate non-exit general overload

We have some indicators about serious non-exit relay general overload going on. We "solved" the exit relay issues by just not using the DNS failure metric anymore (see: #139 (closed) for some analysis of the problem). We might need to tune our metrics that get triggered by non-exits as well.

We should probably use our network-health relays in testing and figuring out what is going on.

/cc @dgoulet