souphttpsrc: Implement soup session sharing
Submitted by Sebastian Dröge @slomo
Link to original bug (#780140)
Description
Also needs some patches for adaptivedemux to make use of that, but with that
it's then possible to use exactly the same connection for the initial
manifest, any variant manifests and the fragments. Which speeds up
time-to-play a lot especially if using HTTPS where every connection setup is
very expensive.