Rename SlabMemoryPool

[JamesNK]

The SlabMemoryPool no longer use slabs - #30732 - and should be renamed.

Possible new name: PinnedBlockMemoryPool

@davidfowl

Thanks for contacting us.
We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. Because it's not immediately obvious that this is a bug in our framework, we would like to keep this around to collect more feedback, which can later help us determine the impact of it. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.