Confirm a record has not already been destroyed before decrementing
counter cache

At present, calling destroy multiple times on the same record results
in the belongs_to counter cache being decremented multiple times. With
this change the record is checked for whether it is already destroyed
prior to decrementing the counter cache.
Merge pull request #10489 from greenriver/ar_counter_cache_multiple_destroy

Confirm a record has not already been destroyed before decrementing counter cache

Conflicts:
	activerecord/CHANGELOG.md
Improve CHANGELOG entry [ci skip]