Partition the document cache

An important problem we have currently and which is illustrated by XWIKI-18947 is that a background thread which load a lot of documents can mess with the whole instance through the document cache by filling it.

One idea to limit this problem a lot would be to modify the way we deal with the documents cache so that a document added to the cache in an HTTP thread (like /view/ action) has priority over other those which were accessed through a background thread when the cache is full. That way a background thread could not make the document cache unusable for the whole instance anymore.

Doing exactly that might not be easy and might depend a lot on Infinispan internals, but at worst we could implement something similar by jingling with two caches instead of one (add a priority cache in which documents accessed during an HTTP thread are added).