Starting from Mongoid 9 and some of 8.x versions we have a combination of features that causes a performance degradation (compared to 7.x):

1. belongs_to associations are now required by default, and this is done with a validation.
2. When a document is persisted, we validate all its associations, including those that were not changed.

This means that when a parent document is saved, validations run on all its children, via has_many and embeds_many.
We believe this behaviour is an overkill for a default. On the other hand, we can imagine
a customer may want to ensure data consistency even with a performance penalty.

Therefore, in 9.x we should introduce a feature flag validate_all_children that is
set to true by default. We filp this flag to false in 10.x.

When the flag is set to false we have the current behavior - slow but everything
is validated. When true, we get back to 7.x behaviour, and validate only children
that are new or changed.