RFR: 8252141: Rename G1YoungRemSetSamplingThread to better reflect its purpose

[stefan.johansson at oracle.com]

Hi,

Please review this rename of the G1 Young RemSet Sampling thread.

Webrev: http://cr.openjdk.java.net/~sjohanss/8252141/00/
JBS: https://bugs.openjdk.java.net/browse/JDK-8252141

Summary
The G1YoungRemSetSamplingThread does more than just remset sampling, it 
is also responsible for the scheduling of periodic GCs. Going forward 
even more taslks will likely be added to this thread and it should be 
renamed. The proposal in this patch is to rename it to G1ServiceThread.

It also includes renaming some members and other related functions.

Testing
Mach5 tier1-3

Thanks,
Stefan