Download HS descriptors

There are still many TODOs here. But at least, what is here is tested. Also, on the live network, it can download an actual descriptor and print it to stderr before panicking.

This is on top of !1115 (merged) !1116 (merged) !1117 (merged). I suggest reviewing those first/separately. The first commit in just this branch is "tor-netdir: Make an import more precise".