A cold plan_reads or read_plan_many searches the block map for all its
blocks at once, a tree level at a time, fetching each level's index nodes
together - at most runtime.meta_read_max_concurrency (default 32) in
flight. Planning 400 scattered ranges over a file of 64 checkpoints went
from 0.56 s to 35 ms on a directory bucket and 4.2 s to 0.23 s on a
general purpose one, with the same requests.