[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

@echo off
rem = """-*-Python-*- script
rem -------------------- DOS section --------------------
rem You could set PYTHONPATH or TK environment variables here
python -x "%~f0" %*
goto exit
 
"""
# -------------------- Python section --------------------
import sys
from os.path import join, dirname, normpath
sys.path.insert(0, normpath(join(dirname(__file__), '..', '..')))
from cubicweb.cwctl import run
run(sys.argv[1:])

DosExitLabel = """
:exit
rem """