tracing: add hooks for gst_pad_send_event_unchecked()
Similar to de30de86, this allows to follow the flow of events as they arrive to a pad rather than only when they are pushed to a peer.
The hook is installed in gst_pad_send_event_unchecked() instead of gst_pad_send_event() because the latter is often omitted: that is the case especifically in gst_pad_push_event_unchecked(), where most event propagation occurs.
This patch also makes use of the new hooks in the log tracer to log the begining and end of the send_event processing.