RFR: JDK-8272112: Arena code simplifications

[Thomas Stuefe]

May I please have reviews for this small change. This is pure code cleanup, issues that were identified by code review when working on JDK-8270308. 

The patch results in less code (excluding the new gtest), bit easier to maintain. I have some smaller functional improvements in the pipeline and would like to get this code cleanup out of the way first.

Changes:

- Made `ChunkPool` an automatic object. We can remove `chunkpool_init()` and instead rely on automatic initialization.

- Instead of having four named pools that need to be handled individually and selected via awkward switch constructs (see the old `Chunk::operator new()` and `::operator delete()`), we replace it with an array and a static selector (`ChunkPool* get_pool_for_size(size_t size)`).

- Removed `ChunkPool::_num_used`. It was nowhere queried, and its usefulness was questionable since these numbers don't include chunks with "special" sizes. The easiest way to obtain the footprint of arena chunks is via NMT.

- Before, we had two places where the backing memory of Chunks was allocated: in `ChunkPool::allocate()` and in `Chunk::operator new()`. The former was unnecessary. I streamlined that: removed the ability to allocate from `ChunkPool::allocate()` and renamed it to `ChunkPool::remove_chunk()`. Now Chunks are only allocated in `Chunk::operator new() `and ChunkPool just serves as Chunk cache.

- Renamed `ChunkPool::free_all_but()` to `ChunkPool::prune()` and removed the argument, made it a method-local constant (it had been a constant before in the caller of that function).

- I added a new gtest to test Chunk allocation and pooling a little.


Functionally, nothing should have changed with this patch, this is pure code grooming.

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Tests: 
- GHA
- I manually tested on 64-bit and 32-bit, with and without UseMallocOnly. I also manually checked that pool usage is as expected.

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Commit messages:
 - arena code grooming

Changes: https://git.openjdk.java.net/jdk/pull/5040/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5040&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272112
  Stats: 148 lines in 3 files changed: 58 ins; 61 del; 29 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5040.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5040/head:pull/5040

PR: https://git.openjdk.java.net/jdk/pull/5040