gst/gstbin.c: Clean up references to the clock provider when disposed or when handling a clock-lost message from it.

Original commit message from CVS:
* gst/gstbin.c: (gst_bin_dispose), (gst_bin_provide_clock_func),
(gst_bin_remove_func), (gst_bin_handle_message_func),
(bin_query_duration_fold), (bin_query_generic_fold):
Clean up references to the clock provider when disposed or when
handling a clock-lost message from it.

Unref sinks when performing a query via gst_iterator_fold, as the
gst_bin_iterate_sinks iterator refs each item. (Fixes #323874)

* gst/gstclock.c: (gst_clock_class_init), (gst_clock_dispose),
(gst_clock_set_master):
Drop our reference to the master clock, if any, when we are disposed.

* gst/gsttypefindfactory.c: (gst_type_find_factory_dispose):
Chain up in dispose.