Defer payment provider logout (bug 1037513)

The API to start a transaction doesn't return
a payment provider because the transaction has not
been created yet. This defers payment provider logout
until right before we redirect to the provider's web
flow.

@muffinresearch r?

There is also a Webpay patch coming shortly.
I wonder if it's something to do with the ``consumeStack: true`` custom code. I haven't had a chance to look into it though.