Allow switching between timeframes

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Metrics should be aggregated into separate buffers to accommodate all the timeframes.
The UI should provide the option to twitch between 1, 5, 15 second intervals.
The setting should be stored in the WS session.
When changing an interval a WS message should be send to the WS server.
After that a batch of metrics should be resent to re-render all the charts.

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Done in #75