[rqlrewrite/schema] Avoid parsing computed relations for each query The RQLRelationRewriter is instanciated for each RQL query, it should avoid parsing computed relations formula by using a cache, which seems rightly located on the instance's schema. This brings a *huge* performance boost to some pages on application with a few computed relations (x4 observed on a client app). Kudos to Adrien, David and Sylvain. Closes #17059828

Twisted < 16.0.0
requests
webtest
cubicweb-blog
cubicweb-file
cubicweb-tag