niimaqsrc: use simpler method of correlating buffers with timestamps

Drop the mutexes and special struct, just create ring buffer of
GstClockTimes. Also rely on callbacks for start/stop of acquisition.

The absolute date-timestamps need to be corrected for the first buffer
timestamp.

sys/niimaq/gstniimaq.h

@@ -66,16 +66,13 @@ struct _GstNiImaqSrc {
   gint64 n_dropped_frames;
   
   guint32** buflist;
-
+  GstClockTime *times;
   INTERFACE_ID iid;
   SESSION_ID sid;
 
   gboolean session_started;
   GstClockTime base_time;
 
-  GSList *timelist;
-  GMutex *frametime_mutex;
-
   GstDateTime *start_time;
   gboolean start_time_sent;
 };