gstpad: Make pad (de)activation atomic

gstpad: Make pad (de)activation atomic

The following could happen previously:
* T1: calls gst_pad_set_active()
* T2: currently (de)activating it
* T1: gst_pad_set_active() returns, caller assumes that the pad has
  completed the requested (de)activation ... whereas it is not
  the case since the actual (de)activation in T2 might still be
  going on.

To ensure atomicity of pad (de)activation, we use a internal
variable (and cond) to ensure only one thread at a time goes through
the actual (de)activation block

https://bugzilla.gnome.org/show_bug.cgi?id=790431