soup-cache-input-stream: fixed a reference cycle

soup-cache-input-stream: fixed a reference cycle

Fixed a reference cycle between SoupCacheInputStream and a
GTask. This reference cycle was preventing the input stream from
being freed when the SoupMessage was cancelled before the stream
was completely read. Furthermore the SoupCache was not able to
complete the pending request because the SoupCacheInputStream was
not aware of the message cancellation and remained waiting forever.

In order to remove the cycle
soup_cache_input_stream_cache(_finish) were removed and the
caching operation finalization is now reported via a signal.
SoupCacheInputStream also implements close_fn which properly
finalizes any pending caching operation.

https://bugzilla.gnome.org/show_bug.cgi?id=682527