Switch the still-failing system test to use it.

Assert that stats have appropriate row count.

Required for executing multiple DML statements w/in context of one
transaction.

Update system tests for DML to exercise this (they previously passed only
one statement).

- Rollback transaction after performing DML.
- Mix DML and batch-style mutations in a single commit.

Also, drop the 'partition' argument to it:  not appropriate to the
usecase.

* Add 'Datatbase.execute_partitioned_dml' method.

* Add system test which exercises PDML. both for UPDATE (with parameter)
  and DELETE.