Add SoupSession:use-thread-context

Add SoupSession:use-thread-context

Add a SoupSession flag telling it to use
g_main_context_get_thread_default() on a per-message basis, rather
than using a single GMainContext for everything. In the simple case,
this is just more glib-like API. In the more complicated case, it
allows synchronously sending one or more messages on a
SoupSessionAsync without running the main GMainLoop.

https://bugs.webkit.org/show_bug.cgi?id=68238