cubicweb #1381328 [sessions] have persistent web sessions [in-progress]

that would avoid sticky cookies hacks when behind a load balancer (and provide vastly smoother balancing)

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patch[connection] remove a dead counter [applied][session] drop .is_repo_in_memory [applied][session] drop session-as-a-context-manager [applied][session] retire session.vreg [applied][session,repo] remove last trace of "cnxprops" [applied][devtools/repotest] use the RepoAccess object [applied][session, repository] provide repo.open_session and move .close reponsibility to session object [applied][repository] remove unused check_session method and its test [applied][web/sessions] the session managers are definitely not components [applied][web] the AuthenticationMAnager is no more an appobject [applied][web] move AbstractAuthManager near its immediate concrete subclass [applied][web] move abstract session manager to web/sessions [applied][session] remove the obsolete timestamp infrastructure [rejected]Use the persistent sessions by default [rejected][schema,session] provide a database CWSession object [rejected][cache,sessions] session persisted through a memcached server [rejected][views/auth] replace a nasty call to locals() with a flag variable [rejected][web/request] use a picklable Counter object for tab index counters [rejected]