Now that indices have a single type by default, we can move to the next step
and identify documents using their `_id` rather than the `_uid`.
One notable change in this commit is that I made deletions implicitly create
types. This helps with the live version map in the case that documents are
deleted before the first type is introduced. Otherwise there would be no way
to differenciate `DELETE index/foo/1` followed by `PUT index/foo/1` from
`DELETE index/bar/1` followed by `PUT index/foo/1`, even though those are
different if versioning is involved.
a72eaa8e0f