Speed up processReadChunk by alternating bytes.IndexByte calls

[JensRantil]

I would not be surprised if you could speed up https://github.com/shraddhaag/1brc/blob/8513d5e70a1bfabbf46ab86a9cb6558bc9805154/main.go#L187C6-L187C22 by alternating calling bytes.IndexByte(buf[lastTokenEnd:], ';'), followed by bytes.IndexByte(buf[lastTokenEnd:], '\n'). bytes.IndexByte function is highly optimized for fast searching in a byte slice. Since it is the inner loop it can have a big impact.

You could probably also skip the looking at the first 1 byte for station name and temperature (since it's at least one character). From my experience not inspecting bytes can be a very efficient way of speeding up parsing.

Also, I would convert from []byte to string only when I need to to avoid any kind of unicode handling.