Make complete checks part of the river task and store the result

[emeschi]

Since the complete check at all levels of merge is information we will want to query constantly and routinely, it make sense to calculate the complete flags (or percentages) for all streams systematically at the time we run the collection of other statistics, in the river plugin. This will at the same time lighten the task of the GUI and/or the server by providing pre-calculated information. For clarity, we might want to run the complete check in a separate river plugin since, unlike the microstate-and-stream-rate plugin, it will have to access the central server for both read and write.