Make zk-atoms actually persist

I spoke too soon before when I said this wasn't an issue. It turns out
that zk-atoms would persist as long as _some_ client was
connected, but otherwise they would disappear. Weirdly, this isn't a
problem with zk-refs.

This commit changes the way that zk-atoms are created in ZooKeeper by
adding `:persistent? true`.
Note that the dataNode (a child of the atom's node) was not being created with `:persistent? true`, even though the [atom node was](https://github.com/liebke/avout/blob/5ea0bb82864c8b1683c3007ff9d0324239bff1ad/src/avout/atoms.clj#L36). In practice, that meant that atoms would persist, but their values would not.