Fix the OOBGC's delta tuning

The prior implementation effectively just set @max_delta=delta.  This approach iteratively tunes @max_delta over time.

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I believe this patch follows the original intent of the delta tuning, but I want to emphasise I haven't been able to properly test this change on a production server yet - we're still on 1.9.3, which AFAICT doesn't update :heap_live_num until GC runs, which means that my live-num-delta is always 0.  So, I understand if you'd rather not merge this just yet.