ewsclient: Correctly cancel pending messages on success

We can not use SOUP_STATUS_NONE with soup_session_cancel_message.
Instead we can use SOUP_STATUS_CANCELLED, as before, and keep track of
the number of pending messages so that we can finish the asynchronous
operation after all the messages have been accounted for.

Fallout from 27e86279bbaff430fe0757f97c97c44d190b7da6

Fixes: https://bugzilla.gnome.org/699553