Avoid returning data for incomplete/partial days for new dashboards

[PatrickNercessian]

As per @bajtos

We are already 'using Grafana's feature called "time shift" to hide the most recent data in the charts. "-1d" for daily values, "-30d" for monthly values' for other dashboards

But 'Maybe we can implement this inside the spark-stats service? We already have some logic to handle date ranges; it understands the concept of “today”. But maybe not “this month”.'
https://github.com/filecoin-station/spark-stats/blob/5e1b373b56acd4ea8817e27e38a82d5e61f959c6/lib/request-helpers.js#L22-L38
Maybe we can modify the SQL queries to something like this: MAX(filter.to, date_trunc('month', now()) - "1 month")

If we never return stats for the current period (day/week/month), then we can also simplify the response cache control header:
https://github.com/filecoin-station/spark-stats/blob/5e1b373b56acd4ea8817e27e38a82d5e61f959c6/lib/request-helpers.js#L75-L87'