overlapping incidents: record in a trailer

[Brian Warner]

In 0.3.1 we changed the way the Incident Reporter worked, so that if a bunch of incident-triggering log events occur at about the same time, they'll all go into a single incident instead of spawning dozens. This was done because we observed (in Tahoe) that these triggers tend to occur in clusters, and having a dozen overlapping incidents didn't give us any more information than a single one (and the extra processing time of the additional incidents was a problem).

But it would be nice if the tools that display and analyze incidents could know about the events that would have triggered their own incidents if only they'd occurred a little bit later. Since the incident header is already written by the time we encounter the extra triggers, maybe we should put them in a 'trailer' dictionary. Or perhaps the event-wrapper dict could get an extra "this is also a trigger" flag.

The web-viewer (and other tools) should mark these secondary triggers with similar highlighting as they mark the primary trigger.