Change memory circuit breaker limit

[ylwu-amzn]

Currently we are using 85% as JVM heap usage limit. If JVM heap usage exceeds 85%, will stop detector. JVM heap usage is not very accurate to evaluate the real system pressure. GC may collect most young generation and system will back to under 85% quickly.

As @kaituo suggested, we can change to old_gen_heap_usage / total_heap > 80% which is more accurate to reflect the memory pressure.