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.
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merge request