On intro failure, Tor2web should build a 3-hop path
This enables two features:
- Tor2web compatibility with Intro Points avoiding being one-hop proxies (#17945 (moved))
- In this case, failure is defined as the inability to connect to an intro point with REASON_TORPROTOCOL (or whatever #17945 (moved) does)
- Tor2web compatibility with ReachableAddresses, in particular:
- IPv6-only Tor2web talking to non-IPv6 intro points
- In this case, failure is defined as the inability to discover a reachable intro point