refactor: Reuse buffers for wide string conversion on Windows

[DataTriny]

Initially extracted from stacked-alloc-refactors but significantly reworked.

Introduce reusable buffers for wide string conversion in the Windows adapter to reduce memory allocations. The initial work used two separate buffers tied to the adapter instance, I chose to go with a thread-local pool of buffers, mostly to avoid acquiring a lock for each node property getter, even if they don't convert strings. Multiple adapters can also share the same buffers. I capped the buffer size to avoid keeping large memory chunks around.

[mwcampbell]

Thanks for taking the time to improve on my version and add test coverage.