Add a helper type to help detect race conditions. There's no performance
or memory use penalty in release builds.
Actually adding the type to various places will be left for future PRs.
(For internal tracking: fixes STAB-852)
Ensure we aren't using multiple allocators with the same list by storing
a pointer to the allocator in debug mode only.
This check is stricter than the bare minimum necessary to prevent
illegal behavior, so CI may reveal certain uses that fail the checks but
don't cause IB. Most of these cases should probably be updated to comply
with the new requirements—we want these types' invariants to be clear.
(For internal tracking: fixes ENG-14987)
