[server] extract creating a new cnxset in a _new_cnxset() helper
So we can move logic specific to _CnxSetPool here.
.. _resources:
Locate resources
----------------
.. automethod:: cubicweb.web.webconfig.WebConfiguration.locate_resource
.. automethod:: cubicweb.web.webconfig.WebConfiguration.locate_doc_file
.. automethod:: cubicweb.web.webconfig.WebConfiguration.locate_all_files
Static files handling
---------------------
.. autoattribute:: cubicweb.web.webconfig.WebConfiguration.static_directory
.. automethod:: cubicweb.web.webconfig.WebConfiguration.static_file_exists
.. automethod:: cubicweb.web.webconfig.WebConfiguration.static_file_open
.. automethod:: cubicweb.web.webconfig.WebConfiguration.static_file_add
.. automethod:: cubicweb.web.webconfig.WebConfiguration.static_file_del