[HPA] Add pool of empty pages to hpa_central

[spredolac]

Motivation
If we already have huge and empty page it may be more efficient to donate it to another hpa shard, then purge and fault it again. This also helps somewhat with virtual address space as one would get the already allocated hpdata object if it is available in the pool before creating another one.
On some of the internal workloads we see improvement in memory, and on some we saw small CPU improvements when using pool.

What
Two commits are plain refactor - moving central related code out of the hpa.c and moving some utility functions into a separate module. Last commit is adding simple page pool which is effectively mutex guarded two lists (purged and non-purged empty folios) that each shard can borrow from.

Testing
Added some unit tests, and also tested in production using internal workloads.

[spredolac]

This explains why certain things are needed in the following commit when this comment is not necessary anymore.

[spredolac]

Will fix this formatting

[guangli-dai]

Discussed offline and concluding the discussion here:

1. The two refactoring commits look good. They isolate different modules of HPA codes more reasonably. We want to take the refactoring commits in first.
2. Adding a central pool is surely what we want in the long term. However, when to donate an empty 2MB units to the pool needs more discussion. If we donate soon, we can theoretically reduce the fragmentation. However, the contention in the central pool also grows rapidly. We want to use bpf to understand the frequency of such contentions and will likely go with a time-based demoting in this case. In short, the pool part needs more experiments and discussions to make the CPU-memory trade-off more efficient as well as configurable.