Logviewer: Augment request.log and access log with additional statistics

[olaf-otto]

It would be useful to provide additional statistics in the logviewer. For instance, the request and access log views may provide:

• The number of requests per second in the time window represented by the excerpt
• The number of concurrent users in the time window represented by the excerpt
• The number of 2xx, 3xx, 4xx and 5xx requests, e.g. in a chart.
• A way to restrict the above statistics to an amount of time available in the view