Shuffle directories and filenames to get rid of historic cruft and to better reflect contrib/production status

* --pid_file is now on command line, instead of in cfg file (better supports startup shell scripts)
* filer now supports done_dir, where processed messages get stored instead of removing them
* sender now can opportunistically wait if the number of event files to process is lower than send_events_limit. Governed by owait_* cfg file options