[repository] move cnxset pool handling to a helper class
The class has responsability to handle connections pool operations in a single
public attribute 'cnxsets'.
On Repository _get_cnxset() and _free_cnxset() are replaced by cnxsets.get()
and cnxsets.release().
Drop multiple access to private attributes and methods from outside of Repository.
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/python-cubicweb-documentation/html/index.html
Files: /usr/share/doc/python-cubicweb-documentation/html/*