Always check the return from isc_refcount_decrement.
Created isc_refcount_decrement_expect macro to test conditionally the return value to ensure it is in expected range. Converted unchecked isc_refcount_decrement to use isc_refcount_decrement_expect. Converted INSIST(isc_refcount_decrement()...) to isc_refcount_decrement_expect.
(cherry picked from commit bde5c763)
Closes #1456 (closed)