Original commit message from CVS:
* gst/gstbin.h:
* gst/gstelement.c: (gst_element_class_init),
(gst_element_set_state), (gst_element_set_state_func):
* gst/gstelement.h:
Pending ABI changes.
GThreadPool in GstBinClass to monitor async state changes.
state_cookie in GstElement to detect concurrent gst/set state.
set_state is now virtual too in case a very complicated element
has to be constructed.