Switch to using date added to ES

[bderenzi]

There is a bug where any unprocessed form that is in the backlog for ES will never be sync'd to a just-completed sync because we use form submission date as opposed to the data added to ES.

The proposal is to switch to using date-added-to-ES so that backlog forms will be sync'd as well.

[snopoke]

Another reason to do this is that it will prevent the tool from getting stuck when there is more than 'batch-size' docs with the same modified on date e.g. cases updated by a large case import.

[snopoke]

dimagi/commcare-hq#24393

[snopoke]

Fixed by #159