Verifies that each action timeout's <step>_<timeout>_fired_at column is one
the timeout can safely own.
AshWorkflow.Transformers.AddAttributes adds the column unless an attribute
with that name already exists, the same as an every's last-fired column.
This verifier rejects two cases:
- an existing attribute with that name whose type is not a datetime, since
AshWorkflow.Changes.RecordEventwrites the firing instant into it; - two action timeouts, or an action timeout and an
every, that resolve to the same column. Step:a_bwith timeout:cand step:awith timeout:b_cboth generate:a_b_c_fired_at, and one's firing would count as the other's.
AshWorkflow.Verifiers.ValidateEvery checks two everys sharing a column.