cubicweb #4875761 Make req.search_state lazy loaded [validation pending]

The goal is to make update_search_state do something only if __mode is present in the form. Otherwise, the session should remain untouched until some code actually access search_state.

done in3.21.0
load left0.000
closed by#1f84295bfe95 [webrequest] Lazy-load search_state
patch[web/request] Restore attribute setting for search_state [applied][webrequest] Lazy-load search_state [applied]