RFC: datastore state of the union & end-to-end batching (rerun-io#1610)

* add batching rfc

* Update design/batching.md

Co-authored-by: Emil Ernerfeldt <[email protected]>

* Update design/batching.md

Co-authored-by: Emil Ernerfeldt <[email protected]>

* Update design/batching.md

Co-authored-by: Emil Ernerfeldt <[email protected]>

* Update design/batching.md

Co-authored-by: Emil Ernerfeldt <[email protected]>

* qa component instances vs. instance keys

* no sticky

* qa are there any special components & non-integer instance keys

* give some clue about cell incompatibility

* temporary constructs

* zstd is already setup

* more planning

* date and links

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Co-authored-by: Emil Ernerfeldt <[email protected]>