db: consider avoiding O(L0 file count) range key iterator construction

Creating a range key iterator must iterate through all the files within L0. We have a couple options for addressing this:

https://github.com/cockroachdb/pebble/pull/3004#pullrequestreview-1690334803

Jira issue: PEBBLE-69

Start by reading the discussion in cockroachdb/pebble#3004, especially the linked pull-request review, and then trace how range key iterator construction examines L0 files. The issue is done only when an agreed approach avoids the current O(L0 file count) construction cost, but no implementation files or tests are named.

Written by the indexing model from the issue text.