grouper: Decrease storage requirements of stream

By cutting short certain packets, we can greatly decrease the amount of
storage required for an event stream.  This patch will cut down on the
amount of data required for storage of event streams.
event-struct: Move to using a file_header

This replaces manual byte-wrangling with a struct evt_file_header atop the
even stream file.  This has a version number associated with it, as well as
one magic number field and several reserved fields.