RFR: 8367993: G1: Speed up ConcurrentMark initialization [v9]

[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 incrementally with two additional commits since the last revision:

 - Reapply "remove commented out code"
   
   This reverts commit d0d1860058f0dae7813c3e5115e2784da8331f3b.
 - Reapply "Stefan J 4"
   
   This reverts commit c5a7e2bb44ce111f8c8d1d7f728f1bf8013475e0.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28723/files
  - new: https://git.openjdk.org/jdk/pull/28723/files/c5a7e2bb..4cf44ec8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28723&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28723&range=07-08

  Stats: 3 lines in 2 files changed: 2 ins; 1 del; 0 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