Snapshot isolation no longer allowed for optimistic Outbox concurrency

[bording]

Pessimistic concurrency control support was added in #439, and it was mentioned that snapshot isolation would not work with this mode.

Transaction isolation level control for the Outbox was added in #524 and shipped in 8.5.0, but this change seems to assume that pessimistic mode is the only mode. Because of this assumption, snapshot isolation support has been removed entirely.

This appears to be a mistake. We should re-enable support for snapshot isolation when running in the optimistic concurrency mode.