[server] add a looping task to free unused cnxsets capture by dead pyro When a pyro client disconnects, we're not necessarily notified so they can hang on to their session/cnxset forever. Don't let them do that. The hacky looping task is only installed when pyro is enabled. This optional activation save this very hacky patch from being rejected. closes #2825380

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"""cubicweb-%(cubename)s application package

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%(longdesc)s

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"""