fix bug where newly added tables were not being added in "replace" mode

http://manage.dimagi.com/default.asp?116123

This is a pretty tricky bug that involves the interplay between what is a sub-optimal workflow of "replace" being delete everything and start from scratch, and `CouchTransaciton` having the same reference to a document being deleted and modified in the same transaction. Which then added `"_deleted": True` to the underlying dict, so the second save was just a noop repeat of the deletion.

A side effect of this is that it also created a bunch of dangling `FixtureDataItems` that point at a nonexistent type. I'm not sure whether that's a problem since all access is via the type, but we should clean them up at some point.