DEBUG: add BUG_ON_STRESS(): a BUG_ON() implemented only when DEBUG_STRESS > 0

DEBUG: add BUG_ON_STRESS(): a BUG_ON() implemented only when DEBUG_STRESS > 0

The purpose of this new BUG_ON is beyond BUG_ON_HOT(). While BUG_ON_HOT()
is meant to be light but placed on very hot code paths, BUG_ON_STRESS()
might be heavy and only used under stress-testing, to try to detect early
that something bad is starting to happen. This one is not even type-checked
when not defined because we don't want to risk the compiler emitting the
slightest piece of code there in production mode, so as to give enough
freedom to the developers.