Ensure unique datapoint ids

[jvansanten]

Quoth @wombaugh:

I have encountered an issue of my own when parsing the distributed elasticc2 alerts: Among these, the dieSourceID which we use to identifiy datapoints can in very rare cases be duplicates of the diaObjectId which is used to identify transients. This affects us since we use the diaObjectId as id when storing a special dp with e.g. redshift information.

Avoid (almost all) id collisions by using the hash of the body as the datapoint id. This is analogous to AmpelAstro/Ampel-ZTF#60, and has the same weaknesses. Since we don't use any information encoded in diaObjectId/diaSourceId (if it exists), this should be a minimal change.