Tor: Switch to bridges on censored networks

[alvasw]

Some users can't connect to Tor because their network censors Tor traffic. Tor seems to bootstrap fine sometimes but after a while those relays get blocked.

Aug 25 21:40:55.000 [warn] 75 connections have failed:
Aug 25 21:40:55.000 [warn]  67 connections died in state handshaking (Tor, v3 handshake) with SSL state SSL negotiation finished successfully in OPEN
Aug 25 21:40:55.000 [warn]  8 connections died in state handshaking (TLS) with SSL state SSLv3/TLS write client hello in HANDSHAKE
Aug 25 21:40:56.000 [warn] Problem bootstrapping. Stuck at 14% (handshake): Handshaking with a relay. (DONE; DONE; count 76; recommendation warn; host *** at ***:***)
Aug 25 21:40:56.000 [warn] 76 connections have failed:
Aug 25 21:40:56.000 [warn]  68 connections died in state handshaking (Tor, v3 handshake) with SSL state SSL negotiation finished successfully in OPEN
Aug 25 21:40:56.000 [warn]  8 connections died in state handshaking (TLS) with SSL state SSLv3/TLS write client hello in HANDSHAKE
Aug 25 21:40:57.000 [warn] Problem bootstrapping. Stuck at 14% (handshake): Handshaking with a relay. (CONNECTRESET; CONNECTRESET; count 77; recommendation warn; host *** at ***:***)
Aug 25 21:40:57.000 [warn] 77 connections have failed:

We can detect these cases, and offer the user to switch to bridges automatically.

Ref: #2653