dbapi: unconditionally commit on any DQL to avoid stale batching
dbapi: unconditionally commit on any DQL to avoid stale batching

On encountering any DQL statements, we MUST flush any previously
batched operation and run them immediately to have the latest data.

Notice after fixing up Cursor.description which revealed tests
that exorcised the path in which DQLs would be run without
the previous statements.

Fixes #213

googleapis/python-spanner-django,googleapis/python-spanner-django