We stop trying to download an md after 8 failed tries

The config var TestingMicrodescMaxDownloadTries specifies the amount of times we are willing to try to download an md before we give up. It's set to 8 on the real network and 80 on the testnet.

This interacts badly with legacy/trac#21969 (moved) since if we fail to fetch a primary guard md more than 8 times we will give up on it and refuse to bootstrap.