[webrequest] Lazy-load search_state
- The update_search_state function will hit the session data only if __mode is
present in the form.
- set_search_state keep the same semantics
- Direct access to search_state in the session data is changed by an access to
the new property.
Closes #4875761
missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules.tmpl)