After the simplification of NXP-19413 (make quotas computation synchronous), the quota code can be simplified a lot.
This includes:
- restore consistent delta updates broken by
NXP-17699(b0ec6a0ec0d7f37fc229c93b674743748c035c4e), - remove unneeded cache now that updates are synchronous,
- avoid spurious saves,
- debug log level is now TRACE for quotas operations.
The code is now also much easier to understand and contained mainly in class DocumentsSizeUpdater.