Use memory_order_acq_rel in isc_refcount_decrement.

Use memory_order_acq_rel in isc_refcount_decrement.

While

if (isc_refcount_decrement() == 1) {	// memory_order_release
	isc_refcount_destroy();		// memory_order_acquire
	...
}

is theoretically the most efficent in practice, using
memory_order_acq_rel produces the same code on x86_64 and doesn't
trigger tsan data races (which use a idealistic model) if
isc_refcount_destroy() is not called immediately.  In fact
isc_refcount_destroy() could be removed if we didn't want
to check for the count being 0 when isc_refcount_destroy() is
called.

https://stackoverflow.com/questions/49112732/memory-order-in-shared-pointer-destructor