soup-message-io: use gio streams rather than SoupSocket

soup-message-io: use gio streams rather than SoupSocket

Use the socket's input/output streams for the base I/O, and add new
SoupBodyInputStream and SoupBodyOutputStream that can be created from
them to handle the body of a single message (including handling
chunked encoding/decoding).

Update chunk-test, which was assuming that the chunk_allocator
callback would never be called if the message had a 0-length body;
that's no longer true.