[web/test] properly reset the request connection related attribute A new request does not have those attribute to None. There have special value that we now properly reinstall.

[web/test] drop two undocumented suspicious regression test Those two test are not documented and use the API in the wrong way. They do not raise exception by chance. the current request signature is:: def request(self, rollbackfirst=False, url=None, headers={}, **kwargs): So the RQL have nothing to do here.

[webrequest] introduce an alternative implementation using the repoapi DBAPI specific parts of ``CubicWebRequestBase`` are extracted in a ``DBAPICubicWebRequestBase`` decicated class and a new ``ConnectionCubicWebRequestBase`` is introduced that provide the same services but using a repoapi.ClientConnection as data source. For now the ``DBAPICubicWebRequestBase`` is still used by default.