Reverb: Ensure that episode references are added BEFORE they are removed.
Reverb: Ensure that episode references are added BEFORE they are removed.

If we don't do this and then this will result in undefined behaviour when the remover selects the item that is getting inserted.

Also make increment references from `InsertCheckpointItem` as not doing this will result in the same issue as above.

PiperOrigin-RevId: 313253798
Change-Id: Ica65a8a788311755f6f8a55ece8a1aa6a6c5bd17

deepmind/reverb,deepmind/reverb,deepmind/reverb,deepmind/reverb