fn pick_candidate_files(
files: Vec<FileHandle>,
max_output_file_size: Option<u64>,
picker: fn(Vec<SortedRun<FileHandle>>, Option<u64>) -> Vec<FileHandle>,
) -> (Vec<FileHandle>, usize)fn pick_candidate_files(
files: Vec<FileHandle>,
max_output_file_size: Option<u64>,
picker: fn(Vec<SortedRun<FileHandle>>, Option<u64>) -> Vec<FileHandle>,
) -> (Vec<FileHandle>, usize)