[multi-sources-removal] Turn ConnectionsSet into simpler ConnectionWrapper also, SqliteConnectionWrapper becomes a subclass. This is allowed since now ConnectionsSet responsability has been reduced to handling the system source only. This changeset also: * drops useless cnxset_set / check_connection api * deprecates former .source / .connection / container api but it renames neither the session's cnxset attribute, nor the related repository method. Related to #2919300

/var/log/cubicweb/*.log {
        weekly
        missingok
        rotate 10
        compress
        delaycompress
        notifempty
        create 640 root adm
        sharedscripts
        postrotate
           if [ -x /usr/sbin/invoke-rc.d ]; then \
              invoke-rc.d cubicweb reload > /dev/null 2>&1; \
           else \
              /etc/init.d/cubicweb reload > /dev/null 2>&1; \
           fi; \
        endscript
}