Consistenly use UNREACHABLE() instead of ISC_UNREACHABLE()
In couple places, we have missed ISC_UNREACHABLE() replacement with UNREACHABLE(). Replace all ISC_UNREACHABLE() calls with UNREACHABLE() and cleanup few more places that we inconsistenly using INSIST(0); without ISC_UNREACHABLE(); or using ISC_UNREACHABLE(); without INSIST(0);
Backport of !6042 (merged)
Edited by Michał Kępień