SoupHTTPRequest: O brave new world!

Kill SoupHTTPInputStream, and have SoupHTTPRequest return the
message's body_istream directly (with a little help from SoupSession
and its subclasses). SoupHTTPRequest works synchronously now as well
(though it's still the case that async only works with
SoupSessionAsync and sync only works with SoupSessionSync).

https://bugzilla.gnome.org/show_bug.cgi?id=591739