config update: ref #93: lots of scanning and release changes

- remove thread timeout because it doesn't do anything anyway
- change from multi-threaded scanning to multi-processed scanning
- start an initial process if there are > early_process_threshold (new config)
- add dead part cleaning as part of processing (in addition to binaries)
- stability improvements for long-running start.py processes
- fix binaries not being deleted during release processing (while retaining cascaded deletes)
- limit the size of group scans by group_scan_limit option (new config)