Consider moving outkey field to command message key

[slominskir]

Currently outkey is an optional field of the command topic message in the value portion of the key=value. We should consider moving it to the key portion (it might be possible for it to remain optional - need to dig a little on optional fields in a key - could be problematic as empty field does not equal channel name, which is the logical default - maybe doesn't matter). The benefit would be that you could have the same PV map to multiple unique keys in a given output topic. You can already do the inverse, where multiple PVs map to the same key in the output topic.