@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

Use this issue as the starting point and first review how @Unique currently handles FAIL and REPLACE. Define done as supporting the proposed IGNORE and UPDATE behaviors, including their ordering, relation-preserving update semantics, and the restriction on multiple UPDATE strategies; the payload names no files or tests.