[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.
