Serialize commands to the PCM

We generally run three status loops which periodically query the PCM devices: the vacuum gauge and the motor controller on the RS-485 bus, and the power/environmental checks to the PCM itself.

The commands are sent over TCP, and should get queued up for serial execution. It may be that the PCM occasionally has trouble with the multiple commands: the xcuActor can help by serializing commands sent to the PCM so that there is only ever one in flight.