[server] extract creating a new cnxset in a _new_cnxset() helper
So we can move logic specific to _CnxSetPool here.
3.26 (1 February 2018)
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New features
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* For ``pyramid`` instance configuration kind, logging is not handled anymore
by CubicWeb but should be configured through ``development.ini`` file
following https://docs.pylonsproject.org/projects/pyramid/en/latest/narr/logging.html.
Backwards incompatible changes
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* CubicWebConfiguration method 'shared_dir' got dropped.