rfcs: new RFC on in-process tenant servers

[knz]

Text of RFC: https://github.com/knz/cockroach/blob/20220720-rfc-in-memory-tenants/docs/RFCS/20220720_dynamic_tenant_servers.md

Some prototype work ongoing here: #84608.

[cockroach-teamcity]

This change is 

[knz]

@lunevalex is going on record elsewhere saying that we should run all dedicated / self-hosted customers using the serverless model, that is, with separate processes for all secondary tenants. This would make this RFC unnecessary and I could simply close this PR altogether.

Thoughts?

[ajstorm]

This would make this RFC unnecessary and I could simply close this PR altogether.

Until we have a plan for something that would replace this, we should both leave this open, and continue working on it.

Imposing an arbitrary limit because larger values haven't been tested is an unusual thing to do. We can use whatever language we want in our docs, and print messages on the command line, etc, but otherwise let people have fun.

I'm fine with that, so long (a) product is fine with it, and (b) we mark it as being in "preview" mode.

[knz]

TFYR!

bors r=stevendanna

[craig[bot]]

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