Deprecate `LockFreePool` implementation in 2.18 (remove from 3.0)

[cowtowncoder]

Due to reported issues wrt Lock-Free pool implementation (temporarily used as the default RecyclerPool for 2.17.0), its use seems dangerous. If we cannot solve (or at least severely mitigate) the problems, it seems necessary to deprecate this implementation, and remove from 3.0.

No decision is yet made but I created this issue so we can consider it.

[cowtowncoder]

/cc @mariofusco @pjfanning @JooHyukKim @franz1981

[JooHyukKim]

(Just connecting things) Issue stemmed out from #1260 (comment).

The issue can also start with "Fix or deprecate" to align with the original comment.

[cowtowncoder]

Deprecated in 2.18; removed from 3.0 (master).