Generate idempotent SQL for SaveChanges when retrying a commit failure

[AndriySvyryd]

If the entity has client-generated key or concurrency token we could generate idempotent commands to make commit failure handling easier.
For updates without concurrency tokens we can use one of the changed values.

[AndriySvyryd]

Related to #4526

[roji]

@AndriySvyryd can you please write a sentence or two on what this means?

[AndriySvyryd]

This is about dealing with transient failures during transaction commit. Retrying the operation again could result in an exception if it did succeed. Arguably the worst case is when inserting rows with store-generated keys as then the operation can be executed twice causing data corruption.
But even in that case we can leverage the values generated during the last attempt and use them in a condition when retrying, so that the operation will be noop if the last attempt succeeded.

[roji]

To clarify for myself: this is about a transient failure during the transaction commit itself, so we have no idea whether the transaction committed or not.