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/tunnel/config.go

@@ -17,8 +17,7 @@ type Config struct {
 	// "tor" and "psiphon" tunnels.
 	Name string
 
-	// Session is the current measurement session. This
-	// field is mandatory.
+	// Session is the mandatory measurement session.
 	Session Session
 
 	// TorArgs contains the optional arguments that you want us to pass