Tracking issue: scalability/performance issues for user-facing analytics

[flyingmutant]

• investigate performance of uniq vs uniqCombined at scale
• api: a way to request data except for very recent (that is very likely to be invalidated in cache) #137
• investigate if going from 1h to e.g. 24h aggregation could help
• looks like e.g. YouTube only allow pre-defined aggregation; we can support some kind of recording rules (think about Tracking issue: Prometheus compatibility MVP #111 compatibility) and handle this blazingly fast. Maybe even integrated with the previous item?
• investigate making tscache persistent