RFR: 8255001: Move G1PeriodicGCTask to its own file

Thu Nov 26 21:07:07 UTC 2020

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`.

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

Commit messages:
 - 8255001: Move G1PeriodicGCTask to its own file

Changes: https://git.openjdk.java.net/jdk/pull/1471/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1471&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255001
  Stats: 207 lines in 6 files changed: 136 ins; 70 del; 1 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