Add ability to process list pre-existing CF log files and then exit once completed.

[hartfordfive]

If some users have one or many existing log files for the Cloudflare Enterprise Log Share, and don't want to necessarily continuously index new log entries, there's no way allow this currently.

A -once flag option could be added and consequently read logs from all the files matching the -log-file-pattern flag, using the filepath.Glob function. The beat process would then exit after processing all files. Also, the -delete-on-complete flag could indicate if the matching log files are to be deleted one processing complete.