perf: add KV read-modify-write primitive

[petermattis]

Forked from #18657 (comment). For operations such as INSERT INTO .... VALUES(...) ON CONFLICT DO UPDATE SET n = n+1 it would be useful to have a KV primitive that encapsulated the n = n + 1 operation into a single round trip.

Yes, it's possible to push the read-modify-write steps down into the KV layer so it can all be done in one round trip. We have a KV Increment command that serves as a proof of concept for this, although it's not hooked up for use from SQL. This would be the ideal solution for your ON CONFLICT DO UPDATE SET x=x+1 (as opposed to UPSERT which just does a blind write of specified values).

[nvanbenschoten]

For more context, see the discussion in #22778.