RFR: 8367993: G1: Speed up ConcurrentMark initialization [v4]

[Leo Korinth]

> This change moves almost all of the ConcurrentMark initialisation from its constructor to the method `G1ConcurrentMark::fully_initialize()`.  Thus, creation time of the VM can be slightly improved by postponing creation of ConcurrentMark. Most time is saved postponing creation of statistics buffers and threads.
> 
> It is not obvious that this is the best solution. I have earlier experimented with lazily allocating statistics buffers _only_. One could also initialise a little bit more eagerly (for example the concurrent mark thread) and maybe get a slightly cleaner change. However IMO it seems better to not have ConcurrentMark "half initiated" with a created mark thread, but un-initialised worker threads.
> 
> This change is depending on the integration of https://bugs.openjdk.org/browse/JDK-8373253.
> 
> I will be out for vacation, and will be back after new year (and will not answer questions during that time), but I thought I get the pull request out now so that you can have a look.

Leo Korinth has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 11 commits:

 - remove commented out code
 - Stefan J 4
 - Stefan J 3
 - Stefan J 2
 - Stefan J 1
 - Merge branch '8373253' into 8367993
 - Merge branch 'master' into _8373253
 - Fixup after comment from Ivan.
 - Merge branch 'master' into _8367993
 - 8367993: G1: Speed up ConcurrentMark initialization
 - ... and 1 more: https://git.openjdk.org/jdk/compare/eee58545...9d9df671

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Changes: https://git.openjdk.org/jdk/pull/28723/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28723&range=03
  Stats: 102 lines in 10 files changed: 54 ins; 25 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/28723.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28723/head:pull/28723

PR: https://git.openjdk.org/jdk/pull/28723