Convert _attachments to stubs

[jhs]

When a transaction is done, Txn returns the new document to the callback, unmodified. The callback basically has the exact document that is in Couch.

The problem is, there might be enormous inline attachments that would be better off converted to stubs. If you naively use the passed doc value and send it back to couch, that will work, but you will be re-sending the inline attachments again.

So a nice option would be {"convert_stubs":true} telling Txn to convert inline attachments to the stub value before passing it to the user's callback.