[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
### Requirements for tests in various cubicweb/**/test directories. ###
## shared by several test folders
cubicweb-card
docutils
Twisted < 16.0.0
webtest
## cubicweb/test
Pygments
mock
#fyzz XXX pip install fails
cubicweb-file
cubicweb-localperms
cubicweb-tag
## cubicweb/devtools/test
flake8
## cubicweb/hooks/test
psycopg2
## cubicweb/pyramid/test
pyramid >= 1.5.0
waitress >= 0.8.9
wsgicors >= 0.3
pyramid_multiauth
## cubicweb/sobject/test
cubicweb-comment