This is a 4th attempt to make counter cache transactional completely.

Past attempts: rails#9236, rails#14849, rails#23357.

All existing counter cache issues (increment/decrement twice, lost
increment) are caused due to updating counter cache on the outside of
the record saving transaction by assigning belongs_to record, even
though assigning that doesn't cause the record saving.

We have the `@_after_replace_counter_called` guard condition to mitigate
double increment/decrement issues, but we can't completely prevent that
inconsistency as long as updating counter cache on the outside of the
transaction, since saving the record is not always happened after that.

We already have handling counter cache after create/update/destroy,

https://github.com/rails/rails/blob/1b90f614b1b3d06b7f02a8b9ea6cd84f15d58643/activerecord/lib/active_record/counter_cache.rb#L162-L189
https://github.com/rails/rails/blob/1b90f614b1b3d06b7f02a8b9ea6cd84f15d58643/activerecord/lib/active_record/associations/builder/belongs_to.rb#L33-L59

so just removing assigning logic on the belongs_to association makes
counter cache transactional completely.

Closes rails#14849.
Closes rails#23357.
Closes rails#31493.
Closes rails#31494.
Closes rails#32372.
Closes rails#33113.
Closes rails#33117
Closes rails#33129.
Closes rails#33458.