* refactor(tunnel): remove nil tunnels hack
This code was originally introduced because a tunnel could be
nil in session.go. I have verified that every invocation of
tunnel.Start is careful to ensure that we have a tunnel name
and that we don't manipulate a nil tunnel.
For this reason, I'd rather remove this tricky bit of code and
further simplify the tunnel code.
Part of https://github.com/ooni/probe/issues/985
* even better docs
2bafb179c3