[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