ParallelRequestsAggregator generates a line plot that shows the number
of parallel requests among the plotted events.
This plot has two issues:
1. It only considers events that are plotted. Events that occur later,
but were started within the plotted time frame are not considered.
2. For performance reasons we are only plotting points when a value
changed. This leads to diagonal lines.
Now we can find writer much faster, because we don't have to execute
a query for documents that match the tags. We can just look up the
documents in the map.
Speedup: 2-4ms -> 0.002-0.01ms
