Set export names to use some kind of uniqueid so we can continue using current methods for bulk download and mosaicking

[nickrsan]

Right now, with SSEBop_stats, we need to set a date range of some kind, and we use that along with a few other user-provided parameters to export names we assume are unique. This is challenging enough with that environment, but won't work at all for a general exporter. We should instead have the user provide a final filename, and for everything in between, we either obtain a unique ID from Earth Engine's task API or if that doesn't work, generate a UUID to use for task tracking