[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
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html