7068240: G1: Long "parallel other time" and "ext root scanning" when running specific benchmark Summary: In root processing, move the scanning of the reference processor's discovered lists to before RSet updating and scanning. When scanning the reference processor's discovered lists, use a buffering closure so that the time spent copying any reference object is correctly attributed. Also removed a couple of unused and irrelevant timers. Reviewed-by: ysr, jmasa

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