RFR: 8239825: G1: Simplify threshold test for mutator refinement

[Kim Barrett]

Please review this change to the handling of "padding" for the threshold
used to decide whether a mutator thread should perform concurrent
refinement.  Rather than doing a slightly tricky (because of potential
overflow) computation every time a mutator thread completes a buffer,
instead perform that computation once and record the result for repeated
use.

CR:
https://bugs.openjdk.java.net/browse/JDK-8239825

Webrev:
https://cr.openjdk.java.net/~kbarrett/8239825/open.00/

Testing:
mach5 tier1-5 along with changes for JDK-8240133 and JDK-8139652. 
Local (linux-x64) hotspot:tier1 with just this change.