Added comment. No functional changes.
Was wrongly filtering out some arrival/departures due to an old or a new
match being far away from a segment. The filter was added to prevent
wrong arrivals/departures when system wrongly jumps a match forward to a
layover. But the situation for Mission Bay was due to a detour and not a
layover stop. Therefore changed the filtering to only happen for
layovers.
Changed to better handling getting no assignment info for a fix in the
AVL feed. With Mission Bay running detours the NextBus system providing
the AVL feed appears to have trouble with keeping the vehicles
predictable. Every once in a while a vehicle will not have an assignment
for an AVL report. This was causing vehicles to disappear from the
Transitime system and sometimes arrival/departure times to not be
determined. The fix was to use old assignment when matching if don't get
assignment from AVL for a limited number of reports.

Also fixed things so that VehicleEvents are generated when a vehicle is
unassigned. This will allow finding such problems as vehicles becomming
temporarily unpredictable much easier to find.