The direct cause for this was the following observation: there was no mechanism
in place to safeguard counted events across evictions, i.e. the following order
of events was not accounted for:
* ingest/digest a bunch of events (PCs correctly updated)
* eviction (PC still correct)
* server/snappea restart (PC reloaded, but based on new events. not correct).
I though about various approaches to fix this (e.g. snapshotting) but in the end
such approaches added even more complexity to the PC mechanism. I decided to first
check how non-performant the SQL route would be, and this PoC seems to say: just
go SQL.
There's also a small semantic change (probably in the direction of what you'd
expect), namely: the periods are no longer 'calendar' periods.
