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

[Albert Yang]

Hi,

I am sending this to both runtime-dev and gc-dev, since the changed code lives in runtime, 
but it's used by GC.

https://bugs.openjdk.java.net/browse/JDK-8252093
http://cr.openjdk.java.net/~ayang/webrevs/8252093/webrev.1/

Both the code and the literature use `alpha`, but its meaning is actually inverted. I 
could include this refactoring in later revision if people feel that it's desirable.

Tested: tier1 (locally) and tier1-3 (mach5)

-- 
/Albert