Enhanced pt-query-digest --resume file

The original pt-query-digest --resume file was simply an offset at which to resume. The enhanced resume file (automatically detected for backwards-compatibility) has a structure like:

start_offset=A
end_offset=Z

When pt-query-digest finishes, it writes

stop_offset=X

to the file to indicate where it actually stopped processing. If stop_offset < end_offset, then the tool did not finish, probably due to --run-time.