Vision msg: Cleanup timestamp

msg/vision_position_estimate.msg

 uint32 id			# ID of the estimator, commonly the component ID of the incoming message
 
-uint64 timestamp_boot		# time of this estimate, in microseconds since system start
-uint64 timestamp_computer	# timestamp provided by the companion computer, in us
+uint64 timestamp_received	# timestamp when the estimate was received
 
 float32 x			# X position in meters in NED earth-fixed frame
 float32 y			# Y position in meters in NED earth-fixed frame