RFR: 8252093: formula used to calculate decaying variance in numberSeq

[Albert Yang]

No significant change is observed for SPECjbb2015 scores with and without this patch, 
using G1, Shenandoah and Z.

GC logs show slightly more GC cycles (before: 357 and after: 389) in ZGC with this patch, 
and no significant change for G1 and Shenandoah.

-- 
/Albert