Stable SHM: atomic performance considerations

[yellowhatter]

Describe the feature

SHM subsystem uses atomics for both buffer headers (u32) and buffer watchdog(u64).

On some platforms 64-bit atomics are not supported (and I guess that library emulates them with Mutex introducing a huge performance drop in contended scenario), and some platforms support even u128 atomics.

I think for the future plan we need to investigate on this issue and our target platforms and maybe use a platform-dependent atomic sizes in SHM. The codebase more or less intends this future improvement.

I place this issue just to highlight this future milestone.