isc_refcount_decrement() uses invalid memory ordering
Currently, isc_refcount_decrement()
uses memory_order_relaxed
which is wrong for substracting the value.
The boost library has an example on reference counting and memory ordering here: https://www.boost.org/doc/libs/1_57_0/doc/html/atomic/usage_examples.html#boost_atomic.usage_examples.example_reference_counters