Add soup_session_get_feature_for_message, remove _get_proxy_resolver

Add soup_session_get_feature_for_message, remove _get_proxy_resolver

Add a cache to soup_session_get_feature(), and remove the separate
auth_manager and proxy_resolver private vars, and just use
soup_session_get_feature() as needed instead. (Partly inspired by the
fact that the cache branch adds yet another specially-tracked
feature.)

Add soup_session_get_feature_for_message() to get a feature and check
soup_message_disables_feature() on it, and use that to replace
soup_session_get_proxy_resolver().