control_allocator: remove direct mixer, add actuator_{motors,servos} instead

msg/actuator_motors.msg

@@ -0,0 +1,9 @@
+# Motor control message
+uint64 timestamp			# time since system start (microseconds)
+uint64 timestamp_sample	    # the timestamp the data this control response is based on was sampled
+
+uint8 NUM_CONTROLS = 8
+float32[8] control # range: [-1, 1], where 1 means maximum positive thrust,
+                   # -1 maximum negative (if not supported by the output, <0 maps to NaN),
+                   # and NaN maps to disarmed (stop the motors)
+