charts: incomplete data on indexing w/ unclean shutdown

[kilpatty]

Currently we calculate chart data by storing all data from a given day in memory until we are ready to write it to the disk, while this works on older blocks, if we restart during this process then it is possible that we loose some of the chart data.

We can probably mitigate this by storing the currently "not-averaged" data in an index and then remove it when its been indexed in the general data.