OTS11 — Preventing calls in a rollback-only transaction

This is related to the discussion on propagating the transaction status in the
transaction context.

Can an OTS implementation prevent outgoing calls within the scope of a
transaction that has been marked rollback only?

Without nailing this down, we can't really say if we can transparently save any
unnecessary operations after the transaction has been set to rollback only.
There don't seem to be any words to this effect in the spec. I'm not sure if
it's a valid optimization at all.