Fix STATEMENT_TAG

[apstndb]

The current STATEMENT_TAG is not yet compatible with Spanner JDBC driver.

https://cloud.google.com/spanner/docs/jdbc-session-mgmt-commands?hl=en#tags

Sets the request tag for the next statement to be executed. Only one tag can be set per statement. The tag doesn't span multiple statements; it must be set on a per statement basis. A request tag can be removed by setting it to the empty string ('').
-- The statement tag property is cleared after each statement execution.

Now STATEMENT_TAG is effective only in the next query.