New features for MX models; race condition; simple TTL converters

- Always release lock in controller_manager (5755b4a). Without this, there is a race condition when the meta controller still waits for controllers.
- New model MX-12W (b00741d)
- List of supported features for each model in DXL_MODEL_TO_PARAMS (4a0e094)
- (Mainly) for MX series: Read current, set acceleration, torque control mode in dynamixel_io (4a0e094)
- Enable "echo readback" for simple one-wire (TTL) converters (b394e58). Simple USB-serial-TTL converters just use a pull-up and a diode after the FT232, echoing back every byte sent by the host. To use these converters, __write_serial must read back everything it sends. This is enabled by an optional argument in dynamixel_io and dynamixel_serial_proxy, or using the ROS parameter ~serial_ports/<port>/readback_echo for controller_manager. Default behavior is unchanged.