Rationale:

• The arena.<i>.reset feature would at a minimum have to assure that quarantine wasn't enabled.
• Quarantine doesn't interact well with thread caches, which means that applications which use explicit thread caches cannot in general use quarantine safely.
• Redzone support complicates memory layout in ways that make it impossible to compute various tables at compile time (small run sizes vary, which impacts quantized run sizes). This can be mitigated, but at considerable engineering complexity cost.