There is an explicit transaction on the connection. The COPY will be part of that
transaction.
ImplicitAtomic
'Normal' auto-commit mode. The entire COPY operation is atomic. If the number of mutations
exceeds any of the transaction limits of Cloud Spanner, the COPY operation will fail.
ImplicitNonAtomic
The COPY operation is executed as a series of (partly parallel) transactions. The COPY
operation is not atomic, and an error halfway the operation can mean that some data was
persisted to the database.
