io/linereader: replace with bufio.Scanner

[empijei]

io/linereader currently implements a functionality that is very similar to bufio.Scanner, but it differs in some subtle ways (e.g. if a token is too long no error is returned and data is just dropped).

This forces callers to check both err and ndiscarted to correctly use it, which we didn't in some cases, leading to issues.

We should replace this with a bufio.Scanner and have a precise policy on discarding lines that are too long.