[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

#!/bin/sh -e
if [ "$1" = "purge" ] ; then
        update-rc.d cubicweb remove >/dev/null
fi
 
#DEBHELPER#
 
exit 0