On intro failure, Tor2web should build a 3-hop path

This enables two features:

• Tor2web compatibility with Intro Points avoiding being one-hop proxies (legacy/trac#17945 (moved))
  • In this case, failure is defined as the inability to connect to an intro point with REASON_TORPROTOCOL (or whatever legacy/trac#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