fix: use created_at for unsynced session history entries

[arielsvg]

See standardnotes/mobile#432

[atmoio]

Hmm, here we are creating new Date(), which is the present date, but in the constructor of pure_payload we are defaulting to new Date(0), which is 1970. Is this intended? So seems like here after this change if updated_at is nil then entry will default to 1970 rather than present.

[arielsvg]

There's a bug with this line in that payload.serverUpdatedAt is always truthy, because the pure_payload constructor always creates a date object. So updated_at can never be new Date().

I don't know what the original intent was, but I think a history entry making up dates is more confusing than helpful. updated_at being equal to the time of construction does not make sense with how the rest of the code treats that kind of field

More broadly speaking I'm not sure setting a payload's updated_at to 1/1/1970 by default is a very accurate way to describe an unsynced payload, but I may be missing something

[atmoio]

I think the intent is that if creating a new entry, the present time is likely the right time for its creation. But this applies only to the offline use case. I suppose as long as you've tested this functionality in the web app while not signed into an account, and it shows the right date, then all is good.