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