fix(spanner): prevent possible panic for Session not found errors

[olavloite]

The Spanner client library could panic if specifically the following situation would occur:

1. Create a StmtBasedReadWriteTransaction
2. Only execute mutations, so that an explicit BeginTransaction RPC is required.
3. The BeginTransaction RPC returns "Session not found"
4. The client library tries to get a new session from the pool.

Step 4 above would panic, because the client library did not set the readOnlyTransaction.sp field for StmtBasedReadWriteTransactions.

Fixes #10385

Also fixes googleapis/go-sql-spanner#262