Add option to return exposure command when shutter close

Per gen2 command-flow :

• Add an optional returnWhenShutterClose argument to the iic exposure command. When this   argument is set, the command will return immediately after the shutters close, allowing the exposure sequence to continue in the background.
• Add iic sps waitForSequenceCompletion command. This command will grab on the command in the background and only return when the entire sequence is fully completed.