RFR: 8358586: ZGC: Combine ZAllocator and ZObjectAllocator [v8]

[Joel Sikström]

> Hello,
> 
> After [JDK-8353184](https://bugs.openjdk.org/browse/JDK-8353184), ZAllocator has essentially become a mirror of ZObjectAllocator, questioning the relevance of ZAllocator. The purpose of this RFE is to combine ZAllocator and ZObjectAllocator into a single class ZObjectAllocator.
> 
> The solution I propose moves the new ZObjectAllocator as a data member in ZHeap, and is accessed solely from ZHeap. ZObjectAllocator stores a number of allocators, one for each age. Instead of storing eden and relocation allocators separately, all allocators are now in a single array, which makes it more straightforward to iterate over all allocators (when retiring pages for example). Since the underlying allocator (called PerAge) needs to be constructed with different ages, we can't use the default constructor/initialization, so I have a solution in-place for dealing with this.
> 
> Undoing an object allocation has been moved in its entirety (not calling into ZObjectAllocator) to ZHeap, where it fits much better.
> 
> Testing:
> * Oracle's tier 1-5

Joel Sikström 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 10 additional commits since the last revision:

 - Merge branch 'master' into JDK-8358586_zallocator_cleanup
 - Rework ZObjectAllocator
 - Private member variables in ZObjectAlloactorImpl
 - Move implementation to .cpp and static-only interface in .hpp
 - Consistent template friend style
 - Friend class ValueObjBlock
 - AllStatic interface with ZObjectAllocators
 - -Wconversion fixes
 - Intermediate alloc_object in ZHeap
 - 8358586: ZGC: Combine ZAllocator and ZObjectAllocator

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/25693/files
  - new: https://git.openjdk.org/jdk/pull/25693/files/4ef788d7..d2b59317

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25693&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25693&range=06-07

  Stats: 27752 lines in 695 files changed: 15142 ins; 8780 del; 3830 mod
  Patch: https://git.openjdk.org/jdk/pull/25693.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25693/head:pull/25693

PR: https://git.openjdk.org/jdk/pull/25693