[datafeed] make cnxset handling of datafeed source more robust
currently we may run in some cases where the session has no more cnxset
depending on errors and parser's handling of the cnxset.
Also, free the cnxset and reacquire it later, letting a chance to other
threads to run.
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/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html