@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

以此 issue 为起点，首先检查 @Unique 当前如何处理 FAIL 和 REPLACE。将支持所提议的 IGNORE 和 UPDATE 行为定义为完成，包括它们的顺序、保持关系的更新语义，以及对多个 UPDATE 策略的限制；payload 未指明任何文件或测试。