Replace moving averages

[fonsinchen]

Instead of constantly visiting all stats to apply moving averages capacities can be constantly incremented without any correction until the link graph jobs are run. A member "start_date" of Edge would tell how long the stats collection is already running.

There should be an additional link graph job which would "compress" all link stats by dividing capacity by a certain factor and setting the start_date to something more recent. It could also handle splitting due to link timeouts.

[fonsinchen]

Done. The compression has to be done inline though, as the links may change while the job is running. However, it's enough to do that about once in a game year.