Persistently cache bridge descriptors
This has code in it to try to use HTTP if-modified-since when refreshing descriptors, to save on downloads. However, our actual downloading client does not yet support this. So this does not yet fix #619 (closed), although it's a big step on the way.
This branch is on top of !828 (merged). The content here is everything after the commit 9424b097 tor-dirmgr: impl AsRef<dyn Error> for Error