fix(engine): break circular dep betwen session and tunnel (#295)

This diff breaks the circular dependency between session and
tunnel, by introducing the concept of early session.

An early session is a session that is able to fetch the psiphon
configuration file _only_ if it's embedded in the binary.

This breaks `miniooni --tunnel=psiphon` for users who have
access to the OONI backend. They are not the users we are
writing this feature for, though, so I think this is reasonable.

At the same time, this opens up the possibility of creating
a psiphon tunnel when constructing a session, which is the
approach I was following in https://github.com/ooni/probe-cli/pull/286.

This work is part of https://github.com/ooni/probe/issues/985.

Once this diff is in, I can land https://github.com/ooni/probe-cli/pull/286.

internal/engine/session.go

@@ -369,13 +369,12 @@ func (s *Session) MaybeStartTunnel(ctx context.Context, name string) error {
 	s.logger.Infof("starting '%s' tunnel; please be patient...", name)
 	tunnel, err := tunnel.Start(ctx, &tunnel.Config{
 		Name:      name,
-		Session:   s,
+		Session:   &sessionTunnelEarlySession{},
 		TorArgs:   s.TorArgs(),
 		TorBinary: s.TorBinary(),
 		TunnelDir: s.tunnelDir,
 	})
 	if err != nil {
-		s.logger.Warnf("cannot start tunnel: %+v", err)
 		return err
 	}
 	// Implementation note: tunnel _may_ be NIL here if name is ""