when writing old data, we hammer cassandra index with metricdef updates

[Dieterbe]

we set the LastUpdate field based on the timestamp of the data, but compare it against wall clock time.
so if a user creates a new series, and starts writing data from a month ago until now, or when writing data just generally lags behind current clock time by more than 2*updateInterval, we will constantly update the index (both in memory and cassandra) with every point that we receive.

maybe something to think about when working on #559

[Dieterbe]

some ideas:

1. we could keep track of the number of points too since we last updated cassandra. and only update if $lastUpdate-time-clause AND update was at least X points ago
2. compare LastUpdate against timestamp of current point, not against wall clock time.

[woodsaj]

closed by #574