gdbus: Avoid blocking on worker thread in connection initialization

I can't see a reason to spin until the worker thread runs, so don't.
This avoids ugly sched_yield() calls that show up in strace and
annoy me; the code is cleaner now too.

We now grab the types needed for the WebKit workaround in the
thread creation area, but only release them when the thread itself
exits.

https://bugzilla.gnome.org/show_bug.cgi?id=651650