GSimpleAsyncResult: push thread context around callback

When an old pre-thread-default-context API that takes an explicit
GMainContext wants to call a gio API, it must call
g_main_context_push_thread_default() before, and
g_main_context_pop_thread_default() after the gio call, so that the
gio method will return its result to the desired GMainContext.

But this fails for methods like g_socket_client_connect_async() that
make a chain of multiple async calls, since the pushed/popped context
will only affect the initial call.

Fix this by having GSimpleAsyncResult itself push/pop the context
around the callback invocation, so that if the callback queues another
async request, it will stay in the same context as the original one.

https://bugzilla.gnome.org/show_bug.cgi?id=646957