`util.iter_start_of_line()` still reads entire lines into memory

[natefoo]

#22276 was an attempt to fix unnecessarily high memory usage during set_meta() for sequence filetypes on files with extremely long lines. Unfortunately, this doesn't work, since upon failing to read an entire line, we immediately fall through to an unchunked fh.readline().

Here's a memray-flamegraph showing this.

[mvdbeek]

Fixed in #22332, thanks for spotting that @natefoo!