RFR: 8254999: Move G1RemSetSampligTask to more appropriate location

[Stefan Johansson]

Please review this change that moves the `Remembered Set Sampling Task` from `g1ServiceThread.cpp` into `g1RemSet.cpp`. This is basically just a move but with a small edition that the task will reschedule itself instead of running if at start of execution the last GC happend more recently than the value of `G1ConcRefinementServiceIntervalMillis`.

This change does not cover moving the timing measurements from the service thread into the task, that will be covered by: 
[JDK-8252645](https://bugs.openjdk.java.net/browse/JDK-8252645)

Testing
Tier1-3 + local verification that the tast is scheduled as expected.

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Commit messages:
 - 8254999: Move G1RemSetSampligTask to more appropriate location

Changes: https://git.openjdk.java.net/jdk/pull/1395/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1395&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254999
  Stats: 196 lines in 5 files changed: 116 ins; 79 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1395.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1395/head:pull/1395

PR: https://git.openjdk.java.net/jdk/pull/1395