Implement mixed date update strategy

[thomas-maschler]

Is your feature request related to a problem? Please describe.
Sometimes GLAD alerts get stuck on UMD side. And we are not able to update alerts on our side.
Right now, we always wait until all tiles for a given date are present before we start the process.
However, when update get stuck, export from GEE are only partially present. In that we should have the option to update Glad alerts using mixed dates for the update, always looking for the latest date for any given tile. We would then trigger the update manually using this strategy