show overload-caused gaps in the event log

[Brian Warner]

When a log-gatherer gets overloaded, the sending application will drop log messages (to avoid unbounded memory usage). We can detect these in the recorded event stream by noticing non-sequential event numbers. 'flogtool dump' and 'flogtool web-viewer' should put a blank line between these events, to remind the user that there is a gap there.

This needs to take multiple-sources into account: only insert a gap if the higest seen event number for that particular source is less than one minus the new number.