Normal queries from the Spanner client use the ExecuteStreamingSql method,
while DML statements use ExecuteSql. This distinction was also built into
spannertest where ExecuteSql would only support DML statements and required
a transaction to be specified. The session pool however uses ExecuteSql to
execute a simple `SELECT 1` query without specifying any transaction. This
would cause a nil pointer dereference.

This PR introduces support for queries in the ExecuteSql method. The current
logic assumes that the statement is a query if the transaction is a single-
use read-only transaction.

Fixes #3639

…to issue-3639