sql,kv,storage: avoid unnecessary value retrievals

[jbowens]

With value separation (cockroachdb/pebble#112), values will commonly be stored externally to sstables. Retrieving a value will commonly require 1 or 2 additional block reads. Current KV API interfaces do not support retrieving just a key, even if query execution only requires a key. Similarly uniqueness checks that find a key will retrieve its values unnecessarily.

Avoiding value retrievals is the simplest form of end-to-end projection push down, and it doesn't suffer from the API complexity that more general forms of projection push down do.

Jira issue: CRDB-50603

[blathers-crl]

Hi @jbowens, please add branch-* labels to identify which branch(es) this C-bug affects.

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