[web] Move request handling logic into cubicweb application. (closes #2200684) We improve http status handling in the process: ``application.publish`` have been renamed to ``application.handle`` to better reflect it's roles. The request object gain a status_out attribute to convey the HTTP status of the response. WSGI and etwist code have been updated. Exception gain status attribute

     1

change_attribute_type('CWCache', 'timestamp', 'TZDatetime')

     2

change_attribute_type('CWUser', 'last_login_time', 'TZDatetime')

     3

change_attribute_type('CWSource', 'latest_retrieval', 'TZDatetime')

     4

drop_attribute('CWSource', 'synchronizing')

     5

add_attribute('CWSource', 'in_synchronization')