Changing pluggable transports (during start-up) in Tor Browser is broken

The fix for legacy/trac#23347 (moved) made bridges work again in tor but did not fully fix the problem/introduced a new issue: changing pluggable transports during start-up of Tor Browser is broken. Steps to reproduce are:

1. Start Tor Browser and select e.g. obfs3 as a transport
2. Before finally connecting, click the Cancel button
3. Reconfigure the censorship bypass settings and choose, e.g. obfs4
4. Resume starting. This time you might get an error (indicating that it's still an obfs3 bridge your tor wants to connect to) and/or just a stalled bootstrap process.

Even worse, after quitting the browser and restarting the start-up process is still broken: it does not proceed and one only gets

Nov 21 11:55:59.000 [notice] Ignoring directory request, since no bridge nodes are available yet.

messages in the terminal.

The first bad commit is 93a8ed3b.