souphttpsrc: Clean up all pending operations from libsoup before unreffing our context

souphttpsrc: Clean up all pending operations from libsoup before unreffing our context

When we cancel connection attempts and similar things, there are still
some operations pending on our main context from the GCancellables. We
should let them all run before unreffing our context, otherwise we leak
file descriptors.

Unfortunately this requires libsoup 2.47.0 or newer as earlier versions
steal our main context from us and we can't use it for cleanup later
without assertions and funny crashes.

Based on a patch by Dmitry Shatrov <shatrov@gmail.com>.

https://bugzilla.gnome.org/show_bug.cgi?id=663944