element: Don't hold state lock all the time while sending an event
element: Don't hold state lock all the time while sending an event

This lock seems to exist only to prevent elements from changing states while
events are being processed. However events are going to be processed
nonetheless in those elements if sent directly via pads, so protection must
already be implemented inside the elements for event handling if it is needed.

As such having the lock here is not very useful and is actually causing
various deadlocks in different situations as described in
https://bugzilla.gnome.org/show_bug.cgi?id=744040

StreamUtils/gstreamer,centricular/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,jpxiong/gstreamer,centricular/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,jpxiong/gstreamer