spanner: add a statement-based way to run read-write transaction.

[hengfengli]

This adds the support to run the read-write transaction in the following way (no retries will be attempted and errors will be directly returned to the caller):

1. NewReadWriteStmtBasedTransaction
2. Execute statement 1
3. Execute statement 2
...
4. Commit or Rollback

Fixes #1971

[olavloite]

A couple of general comments:

1. Statement based transactions also need to be executed in a retry loop, only the user is now responsible for creating that loop one way or another. We should supply a clear example on how to use a statement based transaction, and emphasize the importance of a retry loop. Only checking the commit statement for an Aborted error is not enough, as any statement on the transaction can return with an Aborted error, and should cause the entire transaction to be retried.
2. The Commit and Rollback methods should not be visible to users of the normal Client.ReadWriteTransaction method. Those methods would otherwise cause confusion, as they should not be called when using the normal Client.ReadWriteTransaction method.

[hengfengli]

@olavloite Please take another look. Thanks.

[olavloite]

Thanks for those changes. I'm very happy with the new ReadWriteTransactionStmtBased struct. Would you mind having a look to see if it would be doable to move the BeginTransaction call into that struct as well, and only have something like a NewReadWriteTransactionStmtBased method in client.go? That way, it won't stand out as much to new users who otherwise might start using it, not realizing that it is intended for libraries / ORMs etc.

[hengfengli]

@olavloite Two things I want to mention in my latest change:

1. Rename from extractRetryDelay to ExtractRetryDelay because we have to make it public to users.
2. Add ExampleNewReadWriteStmtBasedTransaction in examples_test.go. This will be shown as an example in godoc.

Thanks for providing these nice suggestions. Please let me know if there are more issues. I am more than happy to fix them.

[olavloite]

There are a couple of comments that now can be removed, but otherwise LGTM.