Heap partitioning using jemalloc arenas.
Keep a set of private arenas, entirely separate from the normal arenas.

When the number of arenas is set to > 1, in some cases the association
between threads and arenas may not be one-to-one, and arenas may be chosen
on a per-allocation round-robin fashion. Because we never want our partitioned
arenas to be accidentally used for other objects, this patch segregates them
entirely.
Proof of concept: allocate Geolocation in a separate arena.