Merge #66436

66436: rfc: distributed token bucket RFC r=RaduBerinde a=RaduBerinde

Link to [RFC text](https://github.com/RaduBerinde/cockroach/blob/distbucket-rfc/docs/RFCS/20210604_distributed_token_bucket.md).

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This is an initial draft.

Design of a subsystem relevant in the multi-tenant setting
("serverless") which rate limits tenant KV operations in conformance
to a budget target.

Release note: None

Co-authored-by: Radu Berinde <[email protected]>