Session recycling issue

We've been seeing some kind of session recycling issue in our unit tests. I'm not quite sure where exactly the problem lies. It could be a spanner emulator bug, or something in the default spanner library. I was unable to recreate the issue using just spanner library and spanner emulator, so I'm suspecting go-sql-spanner.

The error message is the following:

```
ExecContext failed: spanner: code = "FailedPrecondition", desc = "Cannot read or query using a transaction after it has been committed or rolledback.", requestID = "1.4e5b33ee11eb3bc7.1.3.246.1"
```

Or

```
Previously received a different request with this seqno
```

I synthesized regression tests using ai here https://github.com/googleapis/go-sql-spanner/pull/664.

https://github.com/googleapis/go-sql-spanner/issues/626 and https://github.com/googleapis/go-sql-spanner/issues/608 could be due to the same reason.