☂Plugins to promote to stable for v9.0.0

[zwoop]

• cache_range_requests (need docs)
• certifier (or move to examples)
• multiplexer
• prefetch
• remap_purge

@mlibbey Please double check for correctness.

[zwoop]

This list also needs to be vetted on the mailing list.

[zwoop]

Blocks #5457

[SolidWallOfCode]

I definitely want certifier in either experimental or plugins, it's supposed to be usable as is in production.

I think moving multiplexer up is fine as well, that's gotten production use here.

[zwoop]

I'm postponing promotion of remap_stats, it should be rewritten to use e.g. std::unordered_map and C++, and be truly cross platform. See #5737.