retain incoming events in a bucketed sliding window

If archive_events is set to true in the executive's configuration, then all incoming events will be copied to an archive list before processing. The archive list's key contains a timestamp of the current hour, eg "events_archive:2013041916", and is given an expiry of 3 hours by default (overridable by specifying a number of seconds in events_archive_maxage in the executive's configuration.