@Unique: more on conflict options

@Unique currently supports FAIL and REPLACE that are compatible with Sync.

Proposal to add more options (that likely will not work with Sync):

@Unique(onConflict=IGNORE): ignore the offending object (no changes to the existing conflicting object). If there are multiple unique properties in an entity, this strategy is evaluated first:
if the property conflicts, no other properties will be checked for conflicts.

@Unique(onConflict=UPDATE): the offending object overwrites the existing conflicting object while keeping its ID. Thus, all relations pointing to the existing entity stay intact.
This is useful for a "secondary" ID, e.g. a string "ID". Within an entity, this strategy may be used once only (update target would be ambiguous otherwise).

Multiple strategies in an entity are processed in this order: IGNORE, FAIL, REPLACE, UPDATE.

Originally posted by @greenrobot in https://github.com/objectbox/objectbox-java/issues/509#issuecomment-425055199

Utilisez cette issue comme point de départ et examinez d’abord comment @Unique gère actuellement FAIL et REPLACE. Considérez le travail terminé lorsqu’il prend en charge les comportements proposés IGNORE et UPDATE, y compris leur ordre, la sémantique de mise à jour préservant les relations et la restriction concernant plusieurs stratégies UPDATE ; le payload ne nomme aucun fichier ni aucun test.