[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

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etc/init.d

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etc/cubicweb.d

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etc/bash_completion.d

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usr/bin

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usr/share/doc/cubicweb-ctl

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var/log/cubicweb

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var/lib/cubicweb/backup

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var/lib/cubicweb/instances