RFR: 8255001: Move G1PeriodicGCTask to its own file [v3]

[Stefan Johansson]

> After making the service thread task based it is now possible to move the tasks out of `g1ServiceThread.cpp`. The periodic task is moved to its own files and the initialization and registration with the service thread is done while initializing `G1CollectedHeap`.
> 
> No functionality is changed for the task, but it has been split up into a header and a source file. 
> 
> **Testing**
> tier 1-2 + manual testing setting `G1PeriodicGCInterval` through `jcmd`.

Stefan Johansson has updated the pull request incrementally with one additional commit since the last revision:

  Thomas review

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1471/files
  - new: https://git.openjdk.java.net/jdk/pull/1471/files/d5340fe3..9c26482e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1471&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1471&range=01-02

  Stats: 3 lines in 2 files changed: 1 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1471.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1471/head:pull/1471

PR: https://git.openjdk.java.net/jdk/pull/1471