Rethink coalece and purge trigger and default value

[rjuju]

For now

• snapshot occurs every powa.frequency, by default 5 minutes
• aggregation occurs every powa.coalesce snapshot, by default 100, meaning ~ 8h
• purge occurs every 10 (hardcoded) aggregation, meaning almost 4 days

We should add a powa.purge GUC (default 1), and maybe lower default powa.aggregation GUC.

[marco44]

9fcca79ba935766c239c6eb01c11978129d3c64b