Function max_run_bytes 

pub fn max_run_bytes(block_size: u64, max_coalesced_get_bytes: u64) -> u64

Largest byte span one coalesce_runs run may cover: max_coalesced_get_bytes rounded down to a whole number of blocks (at least one). Shared by coalesce_runs below and the fetch-budget reservation/assertion math in range_cache/mod.rs and range_cache/scheduler.rs, so every one of them agrees on what the largest possible run is.