KPI tracker metrics unusable with high transaction count

[abizjak]

Bug Description

The KPI tracker metrics for activity and transaction counts are not usable with high transaction load.

The reason for htis is that we don't compute the metrics on the fly, and thus to display them we have to go through all transactions in a certain period and reconstruct the counts.

We need to change this so that the relevant metrics are computed on the fly and stored, either via materialized views or a separate table.

We can sacrifice some accuracy, e.g., we can just have buckets of 1 day for transaction counts.