DML statements that are executed in auto-commit mode can use either
atomic transactions, or partitioned non-atomic transactions. The
former is bound by the mutation limits in Spanner. The latter may
update/delete any number of rows.

The transaction type that is used to execute DML statements in
auto-commit mode is determined by the connection variable
`autocommit_dml_mode`. This connection variable now supports a
third value. The supported values are:

  - TRANSACTIONAL (default): Uses atomic read/write transactions.
  - PARTITIONED_NON_ATOMIC: Use Partitioned DML for DML statements in auto-commit mode. Use this mode
    to execute DML statements that exceed the transaction mutation limit in Spanner.
  - TRANSACTIONAL_WITH_FALLBACK_TO_PARTITIONED_NON_ATOMIC: Execute DML statements using atomic read/write
    transactions. If this fails because the mutation limit on Spanner has been exceeded, the DML statement
    is retried using a Partitioned DML transaction.