RFR: 8252141: Rename G1YoungRemSetSamplingThread to better reflect its purpose

[Kim Barrett]

> On Sep 1, 2020, at 6:50 AM, stefan.johansson at oracle.com wrote:
> […]
> Updated webrevs:
> Full: http://cr.openjdk.java.net/~sjohanss/8252141/02
> Inc: http://cr.openjdk.java.net/~sjohanss/8252141/01-02

Looks good.