Migrate get_by_ref -> get

None of the other clients have an idiomatic way of doing this yet, but
providing an optional ref to the `get` method feels right to me.  They return
the same sort of thing, you're just providing and additional condition for the
value you want returned.
Migrate put_key_if_none_match -> put_if_absent

Method name copied from go client, defers to put method
Migrate put_key_if_match -> put_if_absent

Using the same method name from the Go client.