fix(cli): read exact chunk len

[emhane]

Bug: reads capped by File, supposed to be capped by user

• Replaces read_buf with read_exact to ensure the whole chunk length is read from file. This will crash otherwise if the whole file length is passed as chunk length.
• Adds tracing
• With this fix, ChunkedFileReader now reads (big) user defined chunks from file, as it was intended to.

BufReader can improve the speed of programs that make small and repeated read calls to the same file or network socket. It does not help when reading very large amounts at once, or reading just one or a few times.

(src: https://doc.rust-lang.org/std/io/struct.BufReader.html)