PVF worker: explore alternative allocators

[mrcnski]

ISSUE

Overview

It may be interesting to explore different allocators for the PVF workers. Some have neat properties relating to determinism or security:

• https://github.com/plasma-umass/Mesh: low-fragmentation allocator, may help to avoid OOM and may be more deterministic.
• https://github.com/GrapheneOS/hardened_malloc: security-focused malloc. (Unfortunately uses the getrandom syscall which I think we want to block.)
• Others?

Of course we would need to benchmark any alternative we consider and make sure any performance degradation is acceptable.

Note: there is work underway on a global allocator wrapper which can be used to gather accurate allocation stats, so we will no longer need tikv_jemalloc_ctl.