Revert "http: Use a global SoupSession rather than per-backend"

This reverts commit 0d32f60776b98ea360f0c0a452feaea4ae172340.

Reverting for now since this has concurrency with libsoup:
https://bugzilla.gnome.org/show_bug.cgi?id=732783
https://bugzilla.gnome.org/show_bug.cgi?id=732925
Revert "http: Remove use of SoupSessionSync/SoupSessionAsync"

This reverts commit 4b76a7e10d76b26929425dc8a392bf0cee246b98.

Reverting for now since this has concurrency with libsoup:
https://bugzilla.gnome.org/show_bug.cgi?id=732783
https://bugzilla.gnome.org/show_bug.cgi?id=732925