tx_volume stats: Produce windowed results, recompute more efficiently

[csillag]

We compute the tx_volume stats in a materialized view in a wasteful manner: Every time we refresh the stats, the numbers are recomputed for every timestamp in the past, even the ones for which we already have the stats computed. It was easy to code but has downsides:

• costs CPU on the DB cluster (no big deal)
• the 5-min-granularity stats are not refreshed every 5 minutes

The logic from dailyActiveAccountsWorker can likely be reused to compute the stats incrementally.

Additional new requirement from frontend: Provide 24h tx volume at every 5-min timestamp (=windowed sum), much like the /{layer}/stats/active_accounts endpoint. Currently, we provide only the total tx volume for each calendar day.

[csillag]

Closed by #477.