RFR: 8254999: Move G1RemSetSamplingTask to more appropriate location [v2]

[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.

Stefan Johansson has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into 8254999-move-sampling-task
 - 8254999: Move G1RemSetSampligTask to more appropriate location

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1395/files
  - new: https://git.openjdk.java.net/jdk/pull/1395/files/e89895a4..4148eabc

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1395&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1395&range=00-01

  Stats: 3439 lines in 93 files changed: 1999 ins; 926 del; 514 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