Add a `max()` function to get the maximum offset.

Often when consuming a log and then writing to a log you may want to know what the maximum offset you've processed so far.

The issue is that sometimes it's not so easy with log formats to determine this. Eg. With a CRLF seperated JSON file where you index into a file with a byte offset and payload length it's a bit tricker to read backwards in the file to the last CRLF.

Also wanting to keep the API simple too.

Start by locating the public log API and its existing offset-related operations. Define where max() belongs and verify that it returns the greatest processed offset for the described log formats while keeping the API simple; no test or file is named in the issue.

Written by the indexing model from the issue text.