Why and where is this a breaking change

commit

[!!!][BUGFIX] Avoid validating instances more often than needed
    
    The GenericObjectValidator uses a static variable to store objects being
    validated. Objects were still validated over and over because at the
    end of isValid() they were detached again.
    
    This change replaces that detach() call with another concept. Now
    any validator implementing the new ObjectValidatorInterface can be
    given an SplObjectStorage to keep track of instances already
    validated.
    
    If you reset that container when it makes sense (before you
    actively validate objects) you can avoid validating single instances
    more than once in one validation run but still validate at various
    points on your logic flow.
    
    This is a breaking change in situations where you would have a deeply
    nested model hierarchy. In onFlush there is an iterator that iterates
    through all entities that have to be persisted. Having an object A that
    has a B, that has a C, the iterator gets a list of 3 entities to go
    through for validation, namely A, B and C.
    But because A has a pointer to B and B to C, validating A means also
    validating B and validating B means also validating C.
    So before the fix there would be a validation for A+B+C, then for B+C, then for C.
    With the fix we now only validate A,B and C once.
    
    Change-Id: I7f437207e27593425b1fbdfc340a442e685a317f
    Fixes: #37363
    Related: #34154
    Releases: 1.1, 1.2