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/cmd/miniooni/main.go

@@ -10,7 +10,7 @@ import (
 func main() {
 	defer func() {
 		if s := recover(); s != nil {
-			fmt.Fprintf(os.Stderr, "%s", s)
+			fmt.Fprintf(os.Stderr, "FATAL: %s\n", s)
 		}
 	}()
 	Main()