(post-release) Monitor disk and ram usage of core 3.5

The new core will bring nice new monitors for the caching and a fair amount of enhancements which is likely to get us rid of the fear of big attachments saturating the ram.

However, we must careful that the disk space does not get flooded. The current prod-app only has 18G free, actually less than the currently available RAM!

A few measures should be made, with checks at the monitors of caches, so as to protect against disk overflowing and calibrating the cache parameters as much as possible.