Add missing invocations of pthreads destructors

Add isc_mutex_destroy() and isc_rwlock_destroy() calls missing from the commits that introduced the relevant isc_mutex_init() and isc_rwlock_init() calls:

• 76bcb4d1
• 15953043
• 857f3bed

None of these omissions affect any hot paths, so they are not expected to cause operational issues; correctness is the only concern here.

All of these were found using the technique implemented in !5631 (closed).

Also add a couple of missing isc_refcount_decrement() and isc_refcount_destroy() calls found while looking at the surrounding code.

See !6033 (merged), !6271 (merged)

Blocks !5631 (closed)