Cleanups in bandwidth_plugin.py

The server-side code in store_bandwidth seems a bit copy-pastish. The in_time and friends is computed 3 times, once thrown away without any use, twice in a very similar way. It would be better to have just one run of computing it and optionally sum the existing data in DB to it by some kind of map trick, or calling the same summing function on all 4 arrays.

Also, consider better way to handle invalid data (for example bucket with max. speed 0) from client. Ignore the one bucket? Or ignore the whole client? Anyway, the current try-except around the whole client is just a hotfix and should be improved. See #33.