Set dirent cache memory globally #1039
Description
The physical memory is globally limited, but we can't limit dirent cache size globally. We should be able to set dirent cache size globally. This is what a reader need to easily avoid wasting memory.
Currently the dirent cache max memory size can be set at ZIM level, but this is not useful for anybody yet and Kiwix readers need to be careful of not just increasing memory usage proportionally to the number of ZIM open.
Also, and this is a subsidiary question, I have no clue how the cache behaves if we hot change the size of the cache.
Annother subsidiary question are the amount of memory concerned. AFAIK, the were relatively small and therefore the topic was not of prime urgency at the time we have revamped the cache system in 2024/25.
This issue is a follow up of kiwix/libkiwix#1265 (comment)
Blocked by #979