Computation cache

[sbidoul]

Some reports can be slow to compute.

When displaying the report, then using the drill-down feature, the recomputation time when navigating back from drill-down is painful.

We considered a drill-down in a new browser tab but that is not possible with Odoo.
#621 opens a pop-up but the ergonomics is poor and further navigation from there loses the benefit.

Related: #593

The compute method of mis.report.instance returns a json that could be cached.

The idea would be that

• the compute method stores the result in a per instance / per context cache
• the compute method returns the cached json if not too old
• we empty the cache on any write on mis.report.instance or related objects (periods, template, kpis), but not on reported data (move lines, etc) - how to do that cleanly?
• a cron purges the cache for old entries