Use two-pass over input data to speed up chart building.

Use two-pass over input data to speed up chart building.

This way, we can look at the top N hotspots after the first parse,
and then quickly iterate over the file again, discarding most lines.
Only the lines for (de-)allocations must be looked at, everything
else can be reused from the first parse.

Now, we won't spend minutes/hours to create chart data, as we can
look at the explicit hotspots which we found once before. No more
costly merging or sorting of the allocation data on every timestamp.