Message Title

As `processedUnits` set happens to be accessed quite often in ValidationContext, it can be split into two separate ones

By doing this, equals method of corresponding processed unit classes can be simplified as well.

one set
# Run complete. Total time: 00:01:55

Benchmark                                                                                   Mode  Cnt    Score     Error   Units
CascadedValidation.testCascadedValidation                                                  thrpt   20  215.283 ±  18.728  ops/ms
CascadedWithLotsOfItemsAndMoreConstraintsValidation.testCascadedValidationWithLotsOfItems  thrpt   20  501.128 ±  68.409   ops/s
CascadedWithLotsOfItemsValidation.testCascadedValidationWithLotsOfItems                    thrpt   20  830.734 ± 107.699   ops/s


Result "org.hibernate.validator.performance.cascaded.CascadedWithLotsOfItemsValidation.testCascadedValidationWithLotsOfItems":
  892.383 ±(99.9%) 14.613 ops/s [Average]
  (min, avg, max) = (854.388, 892.383, 923.493), stdev = 16.829
  CI (99.9%): [877.770, 906.997] (assumes normal distribution)


# Run complete. Total time: 00:01:51

Two sets

Benchmark                                                                                   Mode  Cnt    Score    Error   Units
CascadedValidation.testCascadedValidation                                                  thrpt   20  278.193 ±  4.795  ops/ms
CascadedWithLotsOfItemsAndMoreConstraintsValidation.testCascadedValidationWithLotsOfItems  thrpt   20  652.048 ±  8.913   ops/s
CascadedWithLotsOfItemsValidation.testCascadedValidationWithLotsOfItems                    thrpt   20  892.383 ± 14.613   ops/s