use Symfony Console's progress bar

Here's how far I got with this:

- progress bar works for import
-- width isn't right -- looking at the Symfony code, it appears this should pick up on the terminal width, but it's not
-- there needs to be a newline after the progress bar, otherwise the next message runs on. Again, you'd think this would be something Symfony would do by itself?
- needs to be implemented for rollback as well

I'm leaving this for now, perhaps someone else can move it along a bit further?

@heddn sorry, I don't really follow your comment.

The patch here works. It doesn't rely on any return from the import, but uses an event subscriber to update the Symfony progress bar for each migrated row:

+++ b/src/EventSubscriber/MigrationDrushCommandProgress.php
@@ -0,0 +1,66 @@
+    $events[MigrateEvents::POST_ROW_SAVE][] = ['postRowSave'];

All that this patch needs is some formatting tweaks, and an event subscriber added for rollback.